Clarify docs about limitations of constraint exclusion with partitions

The current wording can confuse the reader about constraint exclusion
being available at query execution, but this only applies to partition
pruning.

Reported-by: Shouyu Luo
Author: David Rowley
Reviewed-by: Chapman Flack, Amit Langote
Discussion: https://postgr.es/m/15629-2ef8b22e61f8333f@postgresql.org

Author: michaelpq

doc/src/sgml/ddl.sgml

@@ -4009,8 +4009,8 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2008-01-01';
    <itemizedlist>
     <listitem>
      <para>
-      Constraint exclusion is only applied during query planning; unlike
-      partition pruning, it cannot be applied during query execution.
+      Constraint exclusion is only applied during query planning, unlike
+      partition pruning, which can also be applied during query execution.
      </para>
     </listitem>