Restore documentation for const/volatile functions [PR107942]

Restore documentation for const/volatile functions [PR107942]

In r5-7698-g8648c55f3b703a I accidentally removed the documentation of
GCC's special interpretation of const/volatile qualifiers on functions
from the function attributes section, thinking this was just a
bit-rotten leftover from old versions of GCC.  PR107942 points out
that this functionality is still present even though the docs are now gone.

I decided this material didn't really belong in the function
attributes discussion, but a new subsection in the general list of GCC
extensions to the C language.  And I agree with the comment in the
issue that we shouldn't really recommend this usage any more.

gcc/ChangeLog
	PR c/107942
	* doc/extend.texi (C Extensions): Add new section to menu.
	(Function Attributes):  Move dangling index entries to....
	(Const and Volatile Functions): New section.