Doc: add information about partition locking

The documentation around locking of partitions for the executor startup
phase of run-time partition pruning wasn't clear about which partitions
were being locked.  Fix that.

Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/CAApHDvp738G75HfkKcfXaf3a8s%3D6mmtOLh46tMD0D2hAo1UCzA%40mail.gmail.com
Backpatch-through: 13

doc/src/sgml/ddl.sgml

@@ -4690,7 +4690,9 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2008-01-01';
        It is possible to determine the number of partitions which were
        removed during this phase by observing the
        <quote>Subplans Removed</quote> property in the
-       <command>EXPLAIN</command> output.
+       <command>EXPLAIN</command> output.  It's important to note that any
+       partitions removed by the partition pruning done at this stage are
+       still locked at the beginning of execution.
       </para>
      </listitem>