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.

diff --git a/man/systemd.unit.xml b/man/systemd.unit.xml
index 2e298ca04f06caaa6318735a288c1bb4512eca9e..1afa600a08fac4434a77a50cff86a582b821e7f1 100644 (file)
--- a/man/systemd.unit.xml
+++ b/man/systemd.unit.xml
@@ -1556,6 +1556,16 @@
                     </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>