RFR: 8297984: Turn on warnings as errors for javadoc

[Magnus Ihse Bursie]

On Thu, 1 Dec 2022 22:20:00 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> 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.)

Since we do not do "apt install graphviz", configure will automatically determine that full docs is impossible. My intent were to keep it that way, since if for some reason "dot" were to be installed on the GHA image, then suddenly the build time would jump from 3 minutes to 60, so I wanted to make it clear that we never want this to happen. 

However, I realize my comment might be misunderstood as if full docs is impossible to do without the cost of rebuilding everything. I'll see if I can clarify it without turning it into an essay about the reasoning.

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

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