Mention that replace_by doesn't free the node

(cherry picked from commit 58a3ea545338eb237ed17993e264b632a53a5e10)

doc/classes/Node.xml

@@ -521,6 +521,7 @@
 			<argument index="1" name="keep_data" type="bool" default="false" />
 			<description>
 				Replaces a node in a scene by the given one. Subscriptions that pass through this node will be lost.
+				Note that the replaced node is not automatically freed, so you either need to keep it in a variable for later use or free it using [method Object.free].
 			</description>
 		</method>
 		<method name="request_ready">