HDFS-16341. Add block placement policy desc

[GuoPhilipse]

Now, we have six block placement policies supported, we can keep the doc updated.No test cases needed.

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 55s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell 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 _
+1 💚	mvninstall	35m 6s		trunk passed
+1 💚	compile	1m 28s		trunk passed with JDK Ubuntu-11.0.11+9-Ubuntu-0ubuntu2.20.04
+1 💚	compile	1m 16s		trunk passed with JDK Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
+1 💚	mvnsite	1m 26s		trunk passed
+1 💚	javadoc	1m 0s		trunk passed with JDK Ubuntu-11.0.11+9-Ubuntu-0ubuntu2.20.04
+1 💚	javadoc	1m 31s		trunk passed with JDK Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
+1 💚	shadedclient	64m 6s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 18s		the patch passed
+1 💚	compile	1m 21s		the patch passed with JDK Ubuntu-11.0.11+9-Ubuntu-0ubuntu2.20.04
+1 💚	javac	1m 21s		the patch passed
+1 💚	compile	1m 11s		the patch passed with JDK Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
+1 💚	javac	1m 11s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	1m 18s		the patch passed
+1 💚	xml	0m 1s		The patch has no ill-formed XML file.
+1 💚	javadoc	0m 53s		the patch passed with JDK Ubuntu-11.0.11+9-Ubuntu-0ubuntu2.20.04
+1 💚	javadoc	1m 21s		the patch passed with JDK Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
+1 💚	shadedclient	25m 6s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	341m 5s		hadoop-hdfs in the patch passed.
+1 💚	asflicense	0m 38s		The patch does not generate ASF License warnings.
		439m 28s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3691/1/artifact/out/Dockerfile
GITHUB PR	#3691
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient codespell xml
uname	Linux 3d68ad25f65d 4.15.0-147-generic #151-Ubuntu SMP Fri Jun 18 19:21:19 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 306c10e
Default Java	Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.11+9-Ubuntu-0ubuntu2.20.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3691/1/testReport/
Max. process+thread count	2221 (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-3691/1/console
versions	git=2.25.1 maven=3.6.3
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[GuoPhilipse]

cc @ayushtkn

[ayushtkn]

LGTM

[GuoPhilipse]

@ayushtkn do you having other advice before merging to the trunk?

[GuoPhilipse]

thanks @ayushtkn for your review ~