clutter: Clarify clutter_stage_get_actor_at_pos docs

When a user moves their cursor the perceived behaviour is that it will pick what is under the cursor. However this isn't how picking works. Picking does a virtual redraw of the screen, so in some cases what gets picked isn't the same as what the user could see. It more represents what will be drawn on the next frame than what is on screen at present.

It's probably unsafe to change these semantics, and they are useful anyway. Just document it better.