Clarify set_process_input/set_process_unhandled_input behaviour with GUI controls

**Your Godot version:**
Godot 2.3.2

**Issue description:**
The behaviour of these methods with GUI controls is not clear with the current description. I was trying to disable the input of a subtree of control nodes when I came into these methods. Right now it says:

> This is not required for GUI controls!

However, this can lead to confusion. It would be great if it suggests what is the intended way to disable input in GUI controls.

**URL to the documentation page (if already existing):**
[set_input_process docs](https://docs.godotengine.org/en/latest/classes/class_node.html#class-node-method-set-process-input)

[set_process_unhandled_input docs](https://docs.godotengine.org/en/latest/classes/class_node.html#class-node-method-set-process-unhandled-input)