Document mobile motion sensors axes

[Nintorch]

This PR documents the axes values for mobile motion sensor methods: Input.get_accelerometer(), Input.get_gravity() and Input.get_gyroscope() (I don't exactly know how to use Input.get_magnetometer(), and googling it doesn't give the results on what the axes are exactly...)

I decided to document these, because I want to make sure that joypad motion sensors ( #111679 ) have the same axes as these methods, and I think it's better for the developers if the axes are documented.

The wording is partially borrowed from here:

godot/thirdparty/sdl/include/SDL3/SDL_sensor.h

Lines 78 to 131 in dec5a37

	/**
	* The different sensors defined by SDL.
	*
	* Additional sensors may be available, using platform dependent semantics.
	*
	* Here are the additional Android sensors:
	*
	* https://developer.android.com/reference/android/hardware/SensorEvent.html#values
	*
	* Accelerometer sensor notes:
	*
	* The accelerometer returns the current acceleration in SI meters per second
	* squared. This measurement includes the force of gravity, so a device at
	* rest will have an value of SDL_STANDARD_GRAVITY away from the center of the
	* earth, which is a positive Y value.
	*
	* - `values[0]`: Acceleration on the x axis
	* - `values[1]`: Acceleration on the y axis
	* - `values[2]`: Acceleration on the z axis
	*
	* For phones and tablets held in natural orientation and game controllers
	* held in front of you, the axes are defined as follows:
	*
	* - -X ... +X : left ... right
	* - -Y ... +Y : bottom ... top
	* - -Z ... +Z : farther ... closer
	*
	* The accelerometer axis data is not changed when the device is rotated.
	*
	* Gyroscope sensor notes:
	*
	* The gyroscope returns the current rate of rotation in radians per second.
	* The rotation is positive in the counter-clockwise direction. That is, an
	* observer looking from a positive location on one of the axes would see
	* positive rotation on that axis when it appeared to be rotating
	* counter-clockwise.
	*
	* - `values[0]`: Angular speed around the x axis (pitch)
	* - `values[1]`: Angular speed around the y axis (yaw)
	* - `values[2]`: Angular speed around the z axis (roll)
	*
	* For phones and tablets held in natural orientation and game controllers
	* held in front of you, the axes are defined as follows:
	*
	* - -X ... +X : left ... right
	* - -Y ... +Y : bottom ... top
	* - -Z ... +Z : farther ... closer
	*
	* The gyroscope axis data is not changed when the device is rotated.
	*
	* \since This enum is available since SDL 3.2.0.
	*
	* \sa SDL_GetCurrentDisplayOrientation
	*/