RFR: 8297984: Turn on warnings as errors for javadoc

Thu Dec 1 23:32:08 UTC 2022

For some reason, we never turned on `-Werror` for javadoc. It is high time to do so.

This PR also fixes a recent issue with the javadoc in JFR that caused a warning, and enables building of javadoc on GHA. (It takes about 3 minutes, as a separate build job.)

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Commit messages:
 - *sigh* Can't build just release, due to limitations in GHA :(
 - Move docs into a separate build step
 - 8297984: Turn on warnings as errors for javadoc

Changes: https://git.openjdk.org/jdk/pull/11468/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11468&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297984
  Stats: 25 lines in 3 files changed: 23 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11468.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11468/head:pull/11468

PR: https://git.openjdk.org/jdk/pull/11468