Doc: Add missing step for pg_stat_progress_cluster

There is a step to track when the new heap is written, but this was
missing in the documentation.

Author: Noriyoshi Shinoda
Discussion: https://postgr.es/m/AT5PR8401MB06447FAE88E1592754E958B8EE640@AT5PR8401MB0644.NAMPRD84.PROD.OUTLOOK.COM
Backpatch-through: 12

Author: michaelpq

doc/src/sgml/monitoring.sgml

@@ -4078,6 +4078,12 @@ SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
        <command>CLUSTER</command> is currently sorting tuples.
      </entry>
     </row>
+    <row>
+     <entry><literal>writing new heap</literal></entry>
+     <entry>
+       <command>CLUSTER</command> is currently writing the new heap.
+     </entry>
+    </row>
     <row>
      <entry><literal>swapping relation files</literal></entry>
      <entry>