manual: Update _DEFAULT_SOURCE. [BZ #22862]

The description of the interplay between feature test macros and
compiler options in the description of _DEFAULT_SOURCE is a little
confusing, and dated, so clarify the situation, and don't assume a
specific value for _DEFAULT_SOURCE.

Also, _DEFAULT_SOURCE is supposed to be defined if none of the C/POSIX
feature test macros are defined, but the condition was lacking a test
for _ISOC11_SOURCE, so that is also addressed.

	[BZ #22862]
	* include/features.h: Add _ISOC11_SOURCE to test for whether
	to define _DEFAULT_SOURCE.
	* manual/creature.texi (_DEFAULT_SOURCE): Improve
	documentation.