Add necessary elaboration to documentation for Node3D::get_parent_node_3d

[MajorMcDoom]

This method returns null if the node has top_level set to true, and as such its behaviour is different from simply calling get_parent() as Node3D. This important detail was not mentioned in the docs.

[Mickeon]

I would've solved this differently in #87440 but it is good to have sooner than later.

[ryevdokimov]

This is true, but this is also a weird function IMO. It's probably too late now, but at least the code that it seems to be used for in the engine (which isn't in that many places) would be better understood if it were more explicit in its intention to use transforms it's dependent on or not.

Like:

Node3D *parent = Object::cast_to<Node3D>(node->get_parent());

if(parent && !parent.is_set_as_top_level()){
   //blah
}

[Mickeon]

Indeed, the question the method asks/answers is "Which Node3D does my transformation directly depend on, if any?"

[Mickeon]

I do approve of this as is, but I do agree it could be worded better.

[akien-mga]

Could you squash the commits? See PR workflow for instructions.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.2.2.
Cherry-picked for 4.1.4.