doc: Move ISO C11 feature notes into POSIX chapters.

doc: Move ISO C11 feature notes into POSIX chapters.

* doc/posix-functions/aligned_alloc.texi: Renamed from
doc/glibc-functions/aligned_alloc.texi.
* doc/posix-functions/quick_exit.texi: Renamed from
doc/glibc-functions/quick_exit.texi.
* doc/posix-headers/uchar.texi: Renamed from
doc/glibc-headers/uchar.texi.
* doc/posix-functions/c16rtomb.texi: Renamed from
doc/glibc-functions/c16rtomb.texi.
* doc/posix-functions/c32rtomb.texi: Renamed from
doc/glibc-functions/c32rtomb.texi.
* doc/posix-functions/mbrtoc16.texi: Renamed from
doc/glibc-functions/mbrtoc16.texi.
* doc/posix-functions/mbrtoc32.texi: Renamed from
doc/glibc-functions/mbrtoc32.texi.
* doc/gnulib.texi: Update.
(Glibc uchar.h): Remove section.
Suggested by Eric Blake.