Document lock level used by ALTER TABLE VALIDATE CONSTRAINT

Backpatch all the way back to 9.6.

Author: Simon Riggs <simon.riggs@enterprisedb.com>
Discussion: https://postgr.es/m/CANbhV-EwxvdhHuOLdfG2ciYrHOHXV=mm6=fD5aMhqcH09Li3Tg@mail.gmail.com

doc/src/sgml/ref/alter_table.sgml

@@ -536,6 +536,9 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       (See <xref linkend="sql-altertable-notes"/> below for an explanation
       of the usefulness of this command.)
      </para>
+     <para>
+      This command acquires a <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
+     </para>
     </listitem>
    </varlistentry>