Improve docs for Thread

An attempt to improve the documentation for Thread
- Adds documentation on how and when the `wait_to_finish` should be used
- Adds a note on what to be careful about when using `wait_to_finish`

(cherry picked from commit 9e2882a98982e4d0a8550feca468b87120515bd2)

doc/classes/Thread.xml

@@ -40,6 +40,8 @@
 			<return type="Variant" />
 			<description>
 				Joins the [Thread] and waits for it to finish. Returns what the method called returned.
+				Should either be used when you want to retrieve the value returned from the method called by the [Thread] or before freeing the instance that contains the [Thread].
+				[b]Note:[/b] After the [Thread] finishes joining it will be disposed. If you want to use it again you will have to create a new instance of it.
 			</description>
 		</method>
 	</methods>