Clarify that `KEY_BACK` is unrelated to the Back button on Android
Clarify that `KEY_BACK` is unrelated to the Back button on Android

This closes #19325.

(cherry picked from commit 359c95156ad545e6978e879d242318177c5127bb)

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