Clarified usage of TreeItem get_next*() and get_prev*() functions.
Clarified usage of TreeItem get_next*() and get_prev*() functions.

(cherry picked from commit df5a9228808497f19b5bddf7b58d2a532cd990d0)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot