doc: Update description of pg_constraint.convalidated

The previous description listed the constraint types that this column
was used for, but that was outdated, since not-valid not-null
constraints are now possible.  So just remove that qualification,
rather than trying to keep it updated.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Robert Treat <rob@xzilla.net>
Discussion: https://www.postgresql.org/message-id/flat/CACJufxFo4yTwzbSZrP%2BzQiR6_M00skoZMFaUnNJCdY6he%3DuQfA%40mail.gmail.com