docs: update documentation for ShowLauncherArgs

docs/config/lua/keyassignment/ShowLauncherArgs.md

@@ -10,6 +10,14 @@ The arguments are a lua table with the following keys:
 * `flags` - required; the set of flags that specifies what to show in the launcher
 * `title` - optional; the title to show in the tab while the launcher is active
 
+{{since('nightly')}}
+
+These additional keys are also available
+* `description` - a string to display when in the default mode. Defaults to:
+  `"Select an item and press Enter=launch  Esc=cancel  /=filter"`
+* `fuzzy_description` - a string to display when in fuzzy finding mode. Defaults to:
+  `"Fuzzy matching: "`
+
 The possible flags are listed below. You must explicitly list each item that you
 want to include in the launcher. If you only specify `"FUZZY"` then you will see
 an empty launcher: