Document that max_worker_processes must be high enough in standby.

The setting values of some parameters including max_worker_processes
must be equal to or higher than the values on the master. However,
previously max_worker_processes was not listed as such parameter
in the document. So this commit adds it to that list.

Back-patch to 9.4 where max_worker_processes was added.

doc/src/sgml/high-availability.sgml

@@ -1993,6 +1993,11 @@ LOG:  database system is ready to accept read only connections
          <varname>max_locks_per_transaction</>
         </para>
        </listitem>
+       <listitem>
+        <para>
+         <varname>max_worker_processes</>
+        </para>
+       </listitem>
       </itemizedlist>
    </para>
 

src/backend/access/transam/xlog.c

@@ -6024,6 +6024,10 @@ do { \
 /*
  * Check to see if required parameters are set high enough on this server
  * for various aspects of recovery operation.
+ *
+ * Note that all the parameters which this function tests need to be
+ * listed in Administrator's Overview section in high-availability.sgml.
+ * If you change them, don't forget to update the list.
  */
 static void
 CheckRequiredParameterValues(void)