Document what user is responsible for disconnecting signals then using Node.remove_child()

**Your Godot version:**
3.2.3.stable
**Issue description:**
Document what user is responsible for disconnecting signals then using Node.remove_child().
https://godotengine.org/qa/68193/delete-child-adding-another-closes-signal-connections-between
Maybe make extra warning about signals created by nodes on initialization.
https://godotengine.org/qa/94687/error-_area_enter_tree-condition-true-removing-child-adding
**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_node.html#class-node-method-remove-child