clarify usage around before-quit-for-update event

docs/api/auto-updater.md

@@ -86,7 +86,9 @@ On Windows only `releaseName` is available.
 
 ### Event: 'before-quit-for-update'
 
-Emitted when `quitAndInstall()` is called. 
+This event is emitted after a user calls `quitAndInstall()`.
+
+When this API is called, the `before-quit` event is not emitted before all windows are closed. As a result you should listen to this event if you wish to perform actions before the windows are closed while a process is quitting, as well as listening to `before-quit`.
 
 ## Methods