RFR: 8325075: Enable -Werror for javadoc to fail on any warnings

[Kevin Rushforth]

Now that the last of the javadoc warnings are fixed or suppressed, it is time to enable `-Werror` so that new javadoc warnings don't creep into our code base. The GHA run was successful.

You can see the `-Werror` doing its job in the following GHA workflow run on my [bad-8325075-javadoc-werror](https://github.com/kevinrushforth/jfx/tree/bad-8325075-javadoc-werror) branch, which has the patch from this PR applied on a base without Andy's fix for the last of the javadoc warnings:

https://github.com/kevinrushforth/jfx/actions/runs/8159524643/job/22304043414

That build fails with the expected error:


2024-03-05T16:23:29.6264357Z /home/runner/work/jfx/jfx/jfx/modules/javafx.web/src/main/java/javafx/scene/web/WebErrorEvent.java:125: warning: no comment
2024-03-05T16:23:29.6265947Z     private final String message;
2024-03-05T16:23:29.6266448Z                          ^
2024-03-05T16:23:29.6267675Z /home/runner/work/jfx/jfx/jfx/modules/javafx.web/src/main/java/javafx/scene/web/WebEvent.java:80: warning: no comment
2024-03-05T16:23:29.6269004Z     private final T data;
2024-03-05T16:23:29.6269667Z                     ^
2024-03-05T16:23:31.7938955Z error: warnings found and -Werror specified
2024-03-05T16:23:31.7939652Z Problems generating Javadoc.
2024-03-05T16:23:31.7940244Z 1 error
2024-03-05T16:23:31.7940590Z 91 warnings

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

Commit messages:
 - 8325075: Enable -Werror for javadoc to fail on any warnings

Changes: https://git.openjdk.org/jfx/pull/1390/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1390&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325075
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1390.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1390/head:pull/1390

PR: https://git.openjdk.org/jfx/pull/1390