Fix INSERT OVERRIDING USER VALUE behavior

Fix INSERT OVERRIDING USER VALUE behavior

The original implementation disallowed using OVERRIDING USER VALUE on
identity columns defined as GENERATED ALWAYS, which is not per
standard.  So allow that now.

Expand documentation and tests around this.

Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Reviewed-by: Vik Fearing <vik@postgresfriends.org>
Discussion: https://www.postgresql.org/message-id/flat/CAEZATCVrh2ufCwmzzM%3Dk_OfuLhTTPBJCdFkimst2kry4oHepuQ%40mail.gmail.com