Fix Javadoc issues, hide more Error Prone warnings

[zbynek]

As the Error Prone upgrade and filtering PRs were merged around the same time, compiling GWT now produces thousands of warnings. This PR adds more suppressios and fixes warnings related to Javadoc.

Fixes #8428

[niloc132]

Can probably close (or at least reference) #8428 in the description

[zbynek]

Can probably close (or at least reference) #8428 in the description

I got a bit closer to that by resolving the remaining issues spotted by Error Prone.

There are still 86 issues reported in console by the Javadoc task (down from 100+).

[niloc132]

Do you want to finish them up, or maybe still link and mark as partial? I'd generally love to get to the point of having tickets for many/most changes just as a practical matter.

[zbynek]

If you don't mind this PR getting bigger, I'll fix them here.

[niloc132]

Go for it - I've read each change (and marked as read) so incremental additions won't be too bad.

[niloc132]

Do we still want to consider this a partial fix? I count 18 warnings in the build related to javadoc, all apply to build/out/user/jakarta-src/com/google/gwt/user/server/rpc/jakarta/XsrfTokenServiceServlet.java - to be fair, the file is copied so we could either fix or just punt and call the rest fixed?

[zbynek]

Description updated to full fix. To get rid of the Jakarta warnings we'd have to do some more replacements in the ant script that builds it, and I think we should not increase ant file complexity before the Maven migration.