doc: Document factorial function
This has existed for a very long time, equivalent to the ! and !!
operators, but it was never documented.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/
6ce1df0e-86a3-e544-743a-
f357ff663f68%402ndquadrant.com