RFR: 8297984: Turn on warnings as errors for javadoc [v2]

[Magnus Ihse Bursie]

> 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.)

Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:

  Try to improve comment wording

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11468/files
  - new: https://git.openjdk.org/jdk/pull/11468/files/20bc7222..a48c8a85

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11468&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11468&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 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