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was used for, but that was outdated, since not-valid not-null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

doc/src/sgml/catalogs.sgml

@@ -2629,7 +2629,6 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       </para>
       <para>
        Has the constraint been validated?
-       Currently, can be false only for foreign keys and CHECK constraints
       </para></entry>
      </row>