Document lock obtained during partition detach

On partition detach, we acquire a SHARE lock on all tables that
reference the partitioned table that we're detaching a partition from,
but failed to document this fact.  My oversight in commit f56f8f8d.
Repair.  Backpatch to 12.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20210325180244.GA12738@alvherre.pgsql

doc/src/sgml/ref/alter_table.sgml

@@ -962,6 +962,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       ties to the table from which it was detached.  Any indexes that were
       attached to the target table's indexes are detached.  Any triggers that
       were created as clones of those in the target table are removed.
+      <literal>SHARE</literal> lock is obtained on any tables that reference
+      this partitioned table in foreign key constraints.
      </para>
     </listitem>
    </varlistentry>