HADOOP-18314. Add some description for PowerShellFencer.

[jianghuazhu]

Description of PR

Some descriptions related to PowerShellFencer are missing here, such as: core-default.xml, HDFSHighAvailabilityWithQJM.md, HDFSHighAvailabilityWithNFS.md, NodeFencer.java.
The purpose of this PR is to perfect them.
Details: HADOOP-18314

How was this patch tested?

Here is mainly to update some documents, for testing, there is not much pressure.

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 56s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 1s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+0 🆗	xmllint	0m 0s		xmllint was not available.
+0 🆗	markdownlint	0m 0s		markdownlint was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+0 🆗	mvndep	14m 55s		Maven dependency ordering for branch
+1 💚	mvninstall	25m 12s		trunk passed
+1 💚	compile	23m 2s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	compile	20m 37s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	checkstyle	4m 25s		trunk passed
+1 💚	mvnsite	4m 32s		trunk passed
+1 💚	javadoc	3m 34s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javadoc	3m 39s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	7m 13s		trunk passed
+1 💚	shadedclient	24m 37s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+0 🆗	mvndep	0m 34s		Maven dependency ordering for patch
+1 💚	mvninstall	2m 33s		the patch passed
+1 💚	compile	22m 10s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javac	22m 10s		the patch passed
+1 💚	compile	20m 38s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	javac	20m 38s		the patch passed
-1 ❌	blanks	0m 0s	/blanks-eol.txt	The patch has 5 line(s) that end in blanks. Use git apply --whitespace=fix <<patch_file>>. Refer https://git-scm.com/docs/git-apply
+1 💚	checkstyle	4m 12s		the patch passed
+1 💚	mvnsite	4m 25s		the patch passed
+1 💚	javadoc	3m 26s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javadoc	3m 37s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	7m 21s		the patch passed
+1 💚	shadedclient	24m 44s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	18m 40s		hadoop-common in the patch passed.
+1 💚	unit	384m 41s		hadoop-hdfs in the patch passed.
+1 💚	asflicense	2m 2s		The patch does not generate ASF License warnings.
		633m 5s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4505/1/artifact/out/Dockerfile
GITHUB PR	#4505
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets xmllint markdownlint
uname	Linux 88e009d83195 4.15.0-112-generic #113-Ubuntu SMP Thu Jul 9 23:41:39 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / f8623a51b2def3497341dc6880c4d5056eb1373a
Default Java	Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4505/1/testReport/
Max. process+thread count	3152 (vs. ulimit of 5500)
modules	C: hadoop-common-project/hadoop-common hadoop-hdfs-project/hadoop-hdfs U: .
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4505/1/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	1m 0s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+0 🆗	xmllint	0m 0s		xmllint was not available.
+0 🆗	markdownlint	0m 0s		markdownlint was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+0 🆗	mvndep	18m 10s		Maven dependency ordering for branch
+1 💚	mvninstall	25m 40s		trunk passed
+1 💚	compile	22m 58s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	compile	20m 38s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	checkstyle	4m 24s		trunk passed
+1 💚	mvnsite	4m 27s		trunk passed
+1 💚	javadoc	3m 35s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javadoc	3m 42s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	7m 9s		trunk passed
+1 💚	shadedclient	24m 17s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+0 🆗	mvndep	0m 33s		Maven dependency ordering for patch
+1 💚	mvninstall	2m 28s		the patch passed
+1 💚	compile	22m 12s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javac	22m 12s		the patch passed
+1 💚	compile	20m 37s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	javac	20m 37s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	4m 11s		the patch passed
+1 💚	mvnsite	4m 28s		the patch passed
+1 💚	javadoc	3m 28s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javadoc	3m 40s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	7m 15s		the patch passed
+1 💚	shadedclient	25m 13s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	19m 4s		hadoop-common in the patch passed.
+1 💚	unit	377m 49s		hadoop-hdfs in the patch passed.
+1 💚	asflicense	1m 58s		The patch does not generate ASF License warnings.
		629m 59s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4505/2/artifact/out/Dockerfile
GITHUB PR	#4505
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets xmllint markdownlint
uname	Linux ba80f3e4b9a7 4.15.0-112-generic #113-Ubuntu SMP Thu Jul 9 23:41:39 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 3f89457
Default Java	Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4505/2/testReport/
Max. process+thread count	2752 (vs. ulimit of 5500)
modules	C: hadoop-common-project/hadoop-common hadoop-hdfs-project/hadoop-hdfs U: .
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4505/2/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[jianghuazhu]

Here are some effects of generating HTML from HDFSHighAvailabilityWithQJM.md.

Can you help review this pr, @goiri .
Thank you very much.