Update _physics_process and _process docs to reflect implementation.

[lavishbehemoth]

Your Godot version:
4.5

Issue description:
The description for _physics_process and _process don't reflect the implementation and it's confusing to devs.
_physics_process docs makes it sound like delta changes based on performance/FPS, which isn't true.
It's also worth mentioning that delta in _process is capped.

URL to the documentation page:
https://docs.godotengine.org/en/4.4/classes/class_mainloop.html#class-mainloop-private-method-physics-process
https://docs.godotengine.org/en/4.4/classes/class_node.html#class-node-private-method-physics-process

Will create PR shortly.