HDFS-16307. Improve HdfsBlockPlacementPolicies docs readability

[GuoPhilipse]

imrove docs readability

[goiri]

@GuoPhilipse thanks for the work.
Can you link it with a JIRA?

[GuoPhilipse]

@GuoPhilipse thanks for the work. Can you link it with a JIRA?

Thanks @goiri ,have just updated

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 43s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	markdownlint	0m 0s		markdownlint was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
			_ trunk Compile Tests _
+1 💚	mvninstall	32m 9s		trunk passed
+1 💚	mvnsite	1m 24s		trunk passed
+1 💚	shadedclient	49m 27s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 12s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	1m 10s		the patch passed
+1 💚	shadedclient	19m 44s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	asflicense	0m 34s		The patch does not generate ASF License warnings.
		73m 20s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3631/1/artifact/out/Dockerfile
GITHUB PR	#3631
Optional Tests	dupname asflicense mvnsite codespell markdownlint
uname	Linux 05f25b7b5fd0 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 / d5cf473
Max. process+thread count	548 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3631/1/console
versions	git=2.25.1 maven=3.6.3
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