get_node() could mention scene unique nodes

**Your Godot version:** v4.2.dev5.official [e3e2528ba]

**Issue description:**
I just found out you can call `nodereference.get_node("%UniqueNode")` from anywhere. I thought scene unique nodes could only be accessed from within the same scene, but you can actually call it from outside of the scene that has the unique node, which is very handy.

However the documentation for get_node() implies that these are the only ways to use it:

There's more 'possible paths' than you're lead to believe.