Document the GDScript debugger not supporting Thread yet

See godotengine#2446.

(cherry picked from commit dec2088)

doc/classes/Thread.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		A unit of execution in a process. Can run methods on [Object]s simultaneously. The use of synchronization via [Mutex] or [Semaphore] is advised if working with shared objects.
+		[b]Note:[/b] Breakpoints won't break on code if it's running in a thread. This is a current limitation of the GDScript debugger.
 	</description>
 	<tutorials>
 		<link title="Using multiple threads">https://docs.godotengine.org/en/latest/tutorials/threads/using_multiple_threads.html</link>