Clarified the expected sensor value when a game controller is at rest

(cherry picked from commit 1a20ccb2897fc9b12ad5242f9128cb6bb60368b1)

include/SDL_sensor.h

@@ -84,7 +84,7 @@ typedef enum
  * 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.
+ * 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