RFR: 8296175: Output warning if generated docs contain diagnostic markers

[Hannes Wallnöfer]

Please review a change to print a terminal message if the documentation generated by `HtmlDoclet` contains any diagnostic marker elements for invalid input. The message is printed after the documentation has been generated and before the error/warning count with the purpose to let the user know that the documentation is not ready to be deployed and can be consulted for locating invalid input. 

Example `javadoc` output containing the new message:


...
Generating out/index.html...
Generating out/help-doc.html...
The generated documentation contains diagnostic markers for invalid input.
3 errors
1 warning

-------------

Commit messages:
 - JDK-8296175: Output warning if generated docs contain diagnostic markers

Changes: https://git.openjdk.org/jdk/pull/18980/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18980&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296175
  Stats: 44 lines in 5 files changed: 32 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/18980.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18980/head:pull/18980

PR: https://git.openjdk.org/jdk/pull/18980