Document UID behavior in ResourceSaver's save function

doc/classes/ResourceSaver.xml

@@ -42,6 +42,7 @@
 				Saves a resource to disk to the given path, using a [ResourceFormatSaver] that recognizes the resource object. If [param path] is empty, [ResourceSaver] will try to use [member Resource.resource_path].
 				The [param flags] bitmask can be specified to customize the save behavior using [enum SaverFlags] flags.
 				Returns [constant OK] on success.
+				[b]Note:[/b] When the project is running, any generated UID associated with the resource will not be saved as the required code is only executed in editor mode.
 			</description>
 		</method>
 	</methods>