pgsql: Improve documentation of signal usage for HAVE_SIGPROCMASK and

Log Message:
-----------
Improve documentation of signal usage for HAVE_SIGPROCMASK and
non-HAVE_SIGPROCMASK cases in pqinitmask().

Modified Files:
--------------
pgsql/src/backend/libpq:
pqsignal.c (r1.38 -> r1.39)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pqsignal.c.diff?r1=1.38&r2=1.39)