Document aclitem functions and operators

Document aclitem functions and operators

aclitem functions and operators have been heretofore undocumented.
Fix that. While at it, ensure the non-operator aclitem functions have
pg_description strings.

Does not seem worthwhile to back-patch.

Author: Fabien Coelho, with pg_description from John Naylor, and significant
refactoring and editorialization by me.
Reviewed by: Tom Lane
Discussion: https://postgr.es/m/flat/alpine.DEB.2.21.1808010825490.18204%40lancre