Document caveats of `OS.get_unique_id()`
Document caveats of `OS.get_unique_id()`

(cherry picked from commit 7350f90c579ce6db0be3cf67175778546459322e)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot