HADOOP-16862. [JDK11] Support JavaDoc.

[aajisaka]

JIRA: https://issues.apache.org/jira/browse/HADOOP-16862

Reference: https://bugs.openjdk.java.net/browse/JDK-8212233

UPDATE FOR THOSE WHO GOOGLED THIS BUG:
If the project uses source/target 8, adding 8 in javadoc configuration should make the project buildable on jdk {11, 12, 13}:

  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-javadoc-plugin</artifactId>
    <configuration>
      <source>8</source>
    </configuration>
     ...
  </plugin>

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Comment
+0 🆗	reexec	1m 36s	Docker mode activated.
		_ Prechecks _
+1 💚	dupname	0m 0s	No case conflicting files found.
+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	27m 23s	trunk passed
+1 💚	compile	0m 21s	trunk passed with JDK Ubuntu-11.0.7+10-post-Ubuntu-2ubuntu218.04
+1 💚	compile	0m 16s	trunk passed with JDK Private Build-1.8.0_252-8u252-b09-1~18.04-b09
+1 💚	mvnsite	0m 20s	trunk passed
+1 💚	shadedclient	46m 35s	branch has no errors when building and testing our client artifacts.
+1 💚	javadoc	0m 19s	trunk passed with JDK Ubuntu-11.0.7+10-post-Ubuntu-2ubuntu218.04
+1 💚	javadoc	0m 18s	trunk passed with JDK Private Build-1.8.0_252-8u252-b09-1~18.04-b09
		_ Patch Compile Tests _
+1 💚	mvninstall	0m 14s	the patch passed
+1 💚	compile	0m 12s	the patch passed with JDK Ubuntu-11.0.7+10-post-Ubuntu-2ubuntu218.04
+1 💚	javac	0m 12s	the patch passed
+1 💚	compile	0m 11s	the patch passed with JDK Private Build-1.8.0_252-8u252-b09-1~18.04-b09
+1 💚	javac	0m 11s	the patch passed
+1 💚	mvnsite	0m 15s	the patch passed
+1 💚	whitespace	0m 0s	The patch has no whitespace issues.
+1 💚	xml	0m 1s	The patch has no ill-formed XML file.
+1 💚	shadedclient	18m 19s	patch has no errors when building and testing our client artifacts.
+1 💚	javadoc	0m 14s	the patch passed with JDK Ubuntu-11.0.7+10-post-Ubuntu-2ubuntu218.04
+1 💚	javadoc	0m 13s	the patch passed with JDK Private Build-1.8.0_252-8u252-b09-1~18.04-b09
		_ Other Tests _
+1 💚	unit	0m 13s	hadoop-project in the patch passed.
+1 💚	asflicense	0m 29s	The patch does not generate ASF License warnings.
		72m 1s

Subsystem	Report/Notes
Docker	ClientAPI=1.40 ServerAPI=1.40 base: https://builds.apache.org/job/hadoop-multibranch/job/PR-2094/1/artifact/out/Dockerfile
GITHUB PR	#2094
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient xml
uname	Linux c455acc53df8 4.15.0-101-generic #102-Ubuntu SMP Mon May 11 10:07:26 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	personality/hadoop.sh
git revision	trunk / 84110d8
Default Java	Private Build-1.8.0_252-8u252-b09-1~18.04-b09
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.7+10-post-Ubuntu-2ubuntu218.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_252-8u252-b09-1~18.04-b09
Test Results	https://builds.apache.org/job/hadoop-multibranch/job/PR-2094/1/testReport/
Max. process+thread count	295 (vs. ulimit of 5500)
modules	C: hadoop-project U: hadoop-project
Console output	https://builds.apache.org/job/hadoop-multibranch/job/PR-2094/1/console
versions	git=2.17.1 maven=3.6.0
Powered by	Apache Yetus 0.12.0 https://yetus.apache.org

This message was automatically generated.

[aajisaka]

This fix is contained by #2098. Closing this.