Mark factorial operator, and postfix operators in general, as deprecated.

Mark factorial operator, and postfix operators in general, as deprecated.

Back-patch key parts of 4c5cf5431 and 6ca547cf7 into stable branches.
I didn't touch pg_description entries here, so it's purely a docs
change; and I didn't fool with any examples either.  The main point
is so that anyone who's wondering if factorial() exists in the stable
branches will be reassured.

Mark Dilger and John Naylor, with some adjustments by me

Discussion: https://postgr.es/m/BE2DF53D-251A-4E26-972F-930E523580E9@enterprisedb.com