ScreenTouchInput.index provides bad or misleading information

**Your Godot version:**
3.2.1

**Issue description:**
The documentation for ScreenTouchInput.index mentions "One index = one finger", which can be interpreted as index containing the index of the touch, such as 0, 1, etc., which isn't the case. Instead, it seems to be a unique number that increments for each event. The value starts rather high and does not map to touch index.

**URL to the documentation page:**
https://docs.godotengine.org/en/stable/classes/class_inputeventscreentouch.html