man: document that RemainAfterExit= doesn't make much sense for repetitive timers

Fixes #3122

diff --git a/man/systemd.timer.xml b/man/systemd.timer.xml
index eb0afee..0d0cccf 100644 (file)
--- a/man/systemd.timer.xml
+++ b/man/systemd.timer.xml
@@ -73,6 +73,12 @@
     <filename>foo.timer</filename> activates a matching service
     <filename>foo.service</filename>. The unit to activate may be
     controlled by <varname>Unit=</varname> (see below).</para>
+
+    <para>Note that in case the unit to activate is already active at the time the timer elapses it is not restarted,
+    but simply left running. There is no concept of spawning new service instances in this case. Due to this, services
+    with <varname>RemainAfterExit=</varname> set (which stay around continously even after the service's main process
+    exited) are usually not suitable for activation via repetitive timers, as they will only be activated once, and
+    then stay around forever.</para>
   </refsect1>
 
   <refsect1>