screen: Add more documentation

objects/screen.c

@@ -20,6 +20,22 @@
  */
 
 /** awesome screen API
+ *
+ * Screen objects can be added and removed over time. To get a callback for all
+ * current and future screens, use `awful.screen.connect_for_each_screen`:
+ *
+ *    awful.screen.connect_for_each_screen(function(s)
+ *        -- do something
+ *    end)
+ *
+ * It is also possible loop over all current screens using:
+ *
+ *    for s, screen do
+ *        -- do something
+ *    end
+ *
+ * Most basic Awesome objects also have a screen property, see `mouse.screen`
+ * `client.screen`, `wibox.screen` and `tag.screen`.
  *
  * Furthermore to the classes described here, one can also use signals as
  * described in @{signals}.