HDFS-15788. Correct the statement for pmem cache to reflect cache persistence support

[PHILO-HE]

Description of PR

Update document to reflect code changes we made previously.

How was this patch tested?

N/A

For code changes:

• [Yes] Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• [ N/A] Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• [N/A ] If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• [ N/A] If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

[rakeshadr]

+1 LGTM, Thanks @PHILO-HE.

Pending build CI

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 45s		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 12s		trunk passed
+1 💚	mvnsite	1m 30s		trunk passed
+1 💚	shadedclient	49m 57s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 15s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	1m 17s		the patch passed
+1 💚	shadedclient	20m 38s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	asflicense	0m 33s		The patch does not generate ASF License warnings.
		74m 51s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3761/1/artifact/out/Dockerfile
GITHUB PR	#3761
Optional Tests	dupname asflicense mvnsite codespell markdownlint
uname	Linux 5d4dae3bfb9c 4.15.0-58-generic #64-Ubuntu SMP Tue Aug 6 11:12:41 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / ecdd45c
Max. process+thread count	547 (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-3761/1/console
versions	git=2.25.1 maven=3.6.3
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[rakeshadr]

Merged the changes to trunk.