Input.get_accelerometer() and Input.get_gravity() need further description

**Your Godot version:**
v4.2.1.stable.official [b09f793f5]

**Issue description:**
I recently learned that these acceleration vectors have two conventions depending on platform.

Android: This is the proper acceleration of the device in units of [m/s²](https://en.wikipedia.org/wiki/Metre_per_second_squared).
iOS: Like that but negated and in units of [g](https://en.wikipedia.org/wiki/Standard_gravity) instead. This is the acceleration of free falling objects relative to the device.

Godot is a mix of both. It uses the negated acceleration in m/s². The docs make no mention of the negation.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_input.html#class-input-method-get-accelerometer
https://docs.godotengine.org/en/stable/classes/class_input.html#class-input-method-get-gravity