Doc: clarify behavior of boolean options in replication protocol commands.

Same idea as ec7e053a9, but applying to the walsender commands
described in protocol.sgml.

Peter Smith

Discussion: https://postgr.es/m/CAHut+PvwjZfdGt2R8HTXgSZft=jZKymrS8KUg31pS7zqaaWKKw@mail.gmail.com

doc/src/sgml/protocol.sgml

@@ -3141,6 +3141,13 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
     </varlistentry>
    </variablelist>
   </para>
+
+  <para>
+   In all the above commands,
+   when specifying a parameter of type <type>boolean</type> the
+   <replaceable class="parameter">value</replaceable> part can be omitted,
+   which is equivalent to specifying <literal>TRUE</literal>.
+  </para>
  </sect1>
 
 <sect1 id="protocol-logical-replication">