Doc: Clarify locks taken when using ALTER TABLE ATTACH PARTITION

Since 898e5e3, this command uses partially ShareUpdateExclusiveLock,
but the docs did not get the call.

Author: Justin Pryzby
Reviewed-by: Amit Langote, Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/20191028001207.GB23808@telsasoft.com
Backpatch-through: 12

Author: michaelpq

doc/src/sgml/ref/alter_table.sgml

@@ -901,6 +901,13 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       the scan of the new partition, it is always skipped when the default
       partition is a foreign table.
      </para>
+
+     <para>
+      Attaching a partition acquires a
+      <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the parent table,
+      in addition to <literal>ACCESS EXCLUSIVE</literal> locks on the table
+      to be attached and on the default partition (if any).
+     </para>
     </listitem>
    </varlistentry>