pgsql: Document return-value conventions used by this implementation,

Log Message:
-----------
Document return-value conventions used by this implementation, per
suggestion from Bruce.

Modified Files:
--------------
pgsql/src/port:
snprintf.c (r1.30 -> r1.31)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/snprintf.c.diff?r1=1.30&r2=1.31)