man: add a note why %U,%h,%s are mostly useless

The reasons were already given in the description of %U, but the
are easy to overlook, and it's better to be explicit to avoid
confusion.

(cherry picked from commit b7a4d8de92)

man/systemd.unit.xml

@@ -1524,6 +1524,16 @@
                     </tbody>
                   </tgroup>
                 </table>
+
+		<para>Please note that specifiers
+		<literal>%U</literal>, <literal>%h</literal>,
+		<literal>%s</literal> are mostly useless when systemd
+		is running in system mode.  PID 1 cannot query the
+		user account database for information, so the
+		specifiers only work as shortcuts for things which are
+		already specified in a different way in the unit
+		file. They are fully functional when systemd is
+		running in <option>--user</option> mode.</para>
         </refsect1>
 
         <refsect1>