add new exec-once keywords (#940)

pages/Configuring/Keywords.md

@@ -32,9 +32,13 @@ You can execute a shell script on:
 - every time the config is reloaded.
 - shutdown of the compositor
 
-`exec-once = command` will execute only on launch
+`exec-once = command` will execute only on launch ([support rules](Dispatchers/#executing-with-rules))
 
-`exec = command` will execute on each reload
+`execr-once = command` will execute only on launch
+
+`exec = command` will execute on each reload ([support rules](Dispatchers/#executing-with-rules))
+
+`execr = command` will execute on each reload
 
 `exec-shutdown = command` will execute only on shutdown