man: Mention Type=oneshot timeout directive

Make the warning for oneshot services (where RuntimeMaxSec= has no
effect) more actionable by pointing to the directive people can use
instead to effectively limit their runtime.

(cherry picked from commit 8c4aa0f)
(cherry picked from commit 468b064)
(cherry picked from commit 017d7fd)

Author: lucaswerkmeister

man/systemd.service.xml

@@ -711,8 +711,8 @@
         <listitem><para>Configures a maximum time for the service to run. If this is used and the service has been
         active for longer than the specified time it is terminated and put into a failure state. Note that this setting
         does not have any effect on <varname>Type=oneshot</varname> services, as they terminate immediately after
-        activation completed. Pass <literal>infinity</literal> (the default) to configure no runtime
-        limit.</para>
+        activation completed (use <varname>TimeoutStartSec=</varname> to limit their activation).
+        Pass <literal>infinity</literal> (the default) to configure no runtime limit.</para>
 
         <para>If a service of <varname>Type=notify</varname>/<varname>Type=notify-reload</varname> sends
         <literal>EXTEND_TIMEOUT_USEC=…</literal>, this may cause the runtime to be extended beyond