doc: Clarify synchronous_standby_names parameter.

The synchronous_standby_names GUC allows specifying num_sync,
the number of synchronous standbys transactions must wait for
replies from. This value must be an integer greater than zero.
This commit updates the documentation to clarify this requirement.

Reported-by: Asphator <asphator@gmail.com>
Discussion: https://postgr.es/m/18663-b02f75cb919f1b60@postgresql.org

doc/src/sgml/config.sgml

@@ -4715,6 +4715,8 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
         wait for replies from,
         and <replaceable class="parameter">standby_name</replaceable>
         is the name of a standby server.
+        <replaceable class="parameter">num_sync</replaceable>
+        must be an integer value greater than zero.
         <literal>FIRST</literal> and <literal>ANY</literal> specify the method to choose
         synchronous standbys from the listed servers.
        </para>