Use @deftypefun for key/specific fns, and add safety notes

for ChangeLog

	* manual/threads.texi (pthread_key_create, pthread_key_delete,
	pthread_getspecific, pthread_setspecific): Format with
	@deftypefun, and add @safety note.
	* manual/signal.texi: Move comments that analyze the above
	functions to their home place.
MT-, AS- and AC-safety docs: manual/users.texi

for ChangeLog

	* manual/users.texi: Document MTASC-safety properties.
MT-, AS- and AC-safety docs: comments for MTASC macros

for ChangeLog

	* manual/macros.texi: Add comments before MTASC-safety macros.
Require check-safety.sh to pass; verify all fns are documented

for  ChangeLog

	* manual/check-safety.sh: Check that each @deftypefn or
	@deftypefun is followed by a @safety remark.
	* manual/Makefile ($(objpfx)stamp-summary): Require
	check-safety.sh to pass.