Godot: _process not called on Node when Node is orphan.

[AllenKll]

Godot version:
3.2.1

OS/device including version:
Windows 10, GPU: potato

Issue description:
I created a new class that inherited from Node. added the _process(delta) function. Instantiated the class using new(). _process was never called.

For debugging purposes, I added the instantiation to the SceneTree, and _process began to be called. Unsure if this is a documentation bug or a code bug. The docs don't indicate that a Node must be part of a tree for _process to be called.

Steps to reproduce:
Create a script with class that inherits from Node, add _process function. Instantiate the class, observe that _process is never called

Minimal reproduction project:
processBug.zip

[Calinou]

I think this is done by design. I suppose we should make it more obvious in the Node class documentation.