Document that SceneTree.call_group() is deferred

(cherry picked from commit 7516ff3)

doc/classes/SceneTree.xml

@@ -23,6 +23,7 @@
 			<description>
 				Calls [code]method[/code] on each member of the given group. You can pass arguments to [code]method[/code] by specifying them at the end of the method call.
 				[b]Note:[/b] [code]method[/code] may only have 5 arguments at most (7 arguments passed to this method in total).
+				[b]Note:[/b] [method call_group] will always call methods with an one-frame delay, in a way similar to [method Object.call_deferred]. To call methods immediately, use [method call_group_flags] with the [constant GROUP_CALL_REALTIME] flag.
 			</description>
 		</method>
 		<method name="call_group_flags" qualifiers="vararg">
@@ -37,6 +38,7 @@
 			<description>
 				Calls [code]method[/code] on each member of the given group, respecting the given [enum GroupCallFlags]. You can pass arguments to [code]method[/code] by specifying them at the end of the method call.
 				[b]Note:[/b] [code]method[/code] may only have 5 arguments at most (8 arguments passed to this method in total).
+				[b]Note:[/b] Group call flags are used to control the method calling behavior. If the [constant GROUP_CALL_REALTIME] flag is present in the [code]flags[/code] argument, methods will be called immediately. If this flag isn't present in [code]flags[/code], methods will be called with a one-frame delay in a way similar to [method call_group].
 			</description>
 		</method>
 		<method name="change_scene">