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 8c4aa0f1c6a78b35712fa6a7acf6d755d0c0bd86)
(cherry picked from commit 468b0646342986c6cc9bd797b4ba189dc488ee8d)

man/systemd.service.xml

@@ -727,8 +727,8 @@
         <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