Document that `File.open_compressed()` can only open files saved by Godot
Document that `File.open_compressed()` can only open files saved by Godot

(cherry picked from commit 5f098d6db6da62deb77d6d555ab67d6cb15f80c2)

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