pgsql: Document that get_attstatsslot/free_attstatsslot only need to be

Log Message:
-----------
Document that get_attstatsslot/free_attstatsslot only need to be passed
valid type information if they are asked to fetch the values part of a
pg_statistic slot; these arguments are unneeded if fetching only the
numbers part.  Use this to save a catcache lookup in btcostestimate,
which is looking like a bit of a hotspot in recent profiling.  Not a
big savings, but since it's essentially free, might as well do it.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        selfuncs.c (r1.189 -> r1.190)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.189&r2=1.190)
    pgsql/src/backend/utils/cache:
        lsyscache.c (r1.127 -> r1.128)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/lsyscache.c.diff?r1=1.127&r2=1.128)