Add explanations for each node's internal processing

**Your Godot version:**
3.2.3.stable.mono.official

**Issue description:**
In the reference page for the Node class, the methods `is_processing_internal()`, `set_process_internal()`, `is_physics_processing_internal()` and `set_physics_process_internal()` describe internal processing as isolated from the `_process()` and `_physics_process()` calls, but in the current GDScript documentation for built-in nodes there is no way of knowing what internal processing actually _does_ for each of the built-in nodes, and so when extending these classes, the effects of turning on and off processing may become unclear. A few examples are Area2D or AnimationPlayer.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_node.html#class-node-method-is-processing-internal