Minor corrections for ALTER TYPE ADD VALUE IF NOT EXISTS patch.

Minor corrections for ALTER TYPE ADD VALUE IF NOT EXISTS patch.

Produce a NOTICE when the label already exists, for consistency with other
CREATE IF NOT EXISTS commands.  Also, fix the code so it produces something
more user-friendly than an index violation when the label already exists.
This not incidentally enables making a regression test that the previous
patch didn't make for fear of exposing an unpredictable OID in the results.
Also some wordsmithing on the documentation.