Document that get_attstatsslot/free_attstatsslot only need to be passed

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.