[Bug bootstrap/33824] Improve configuration documentation

[lhyatt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33824
Bug 33824 depends on bug 12407, which changed state.

Bug 12407 Summary: Need to support configure --disable-languages
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12407

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED