Feedback on page SDL3/CategoryThread

[icculus]

This page does something clever:

SDL thread management routines.

<!-- END CATEGORY DOCUMENTATION -->

This part of the SDL API handles management of threads, but an app also will
need locks to manage thread safety. Those pieces are in [CategoryMutex](CategoryMutex).

Only the first part gets bridged to the headers, and the second part has a wiki-specific link by exploiting a bug/limitation in wikiheaders...so I'm hesitant to change this, but it might be nice to have the note about mutexes bridged to the headers.