Extend documentation for Antrun execution to fix javadocs

Also update the internal review ID of the OpenJDK bug I created, because
previously I had created a feature request, which has remained
unanswered. But actually, I think it is rather a regression bug, having
found out that before JDK 16 the behaviour was exactly how the feature
request describes it.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

pom.xml

@@ -472,14 +472,20 @@
 					<executions>
 						<!--
 							Remove javadoc version number from HTML comment to keep website diffs small.
+							  - In JDKs 8 to 9, the javadoc version was not added to the comment string at all,
+							    see com.sun.tools.doclets.formats.html.markup.HtmlDocWriter::getGeneratedBy.
+							  - In JDKs 10 to 15, the version number was suppressed together with the timestamp in '-notimestamp'
+							    mode, see jdk.javadoc.internal.doclets.formats.html.markup.Head::getGeneratedBy.
+							  - Since JDK 16, the version is generated into the comment string without regard of the timestamp
+							    setting. :-/
 
 							Attention: This has no effect on javadocs generated by the 'jar' goal, because they are archived while
 							Maven Javadoc runs, i.e. before this plugin execution kicks in. However, it cleans the javadocs before
 							Maven Resources copies them to aj-build/dist/docs/doc. I.e., they will be clean on the website, if copied
 							from that folder, and also in the installer.
 
-							TODO: In the future, maybe this execution or the whole Antrun plugin can go away, after a JDK feature
-							  request I just created (internal review ID 9076583) has been implemented and back-ported into all JDK
+							TODO: In the future, maybe this execution or the whole Antrun plugin can go away, after a JDK bug
+							  request I just created (internal review ID 9076602) has been implemented and back-ported into all JDK
 							  versions we support for javadoc generation. But if that ever happens, it might take a while.
 						-->
 						<execution>