man: add a note that NULL "s" arg for sd_bus_message_append

man/sd_bus_message_append.xml

@@ -169,6 +169,11 @@
       </tgroup>
     </table>
 
+    <para>For types "s" and "g" (unicode string or signature), the pointer may be
+    <constant>NULL</constant>, which is equivalent to an empty string. See
+    <citerefentry><refentrytitle>sd_bus_message_append_basic</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    for the precise interpretation of those and other types.</para>
+
   </refsect1>
 
   <refsect1>