manual: Document missing feature test macros.

Several feature test macros are documented in features.h but absent in
the manual, and some documented macros accept undocumented values.
This commit updates the manual to mention all the accepted macros,
along with any values that hold special meaning.

	* manual/creature.texi (_POSIX_C_SOURCE): Document special
	values of 199606L, 200112L, and 200809L.
	(_XOPEN_SOURCE): Document special values of 600 and 700.
	(_ISOC11_SOURCE): Document macro.
	(_ATFILE_SOURCE): Likewise.
	(_FORTIFY_SOURCE): Likewise.