The glibc manual uses special annotations to include functions
in the summary chapter. These annotations were missing from the
functions in the threads chapter. This patch adds those special
markers and in turn adds these functions to the summary chapter.
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.