Mention that grab_focus is more reliable deferred

(cherry picked from commit 188d5593e1f080181a6e0302b7d54416d2cd96e8)

doc/classes/Control.xml

@@ -395,6 +395,7 @@
 			<return type="void" />
 			<description>
 				Steal the focus from another control and become the focused control (see [member focus_mode]).
+				[b]Note[/b]: Using this method together with [method Object.call_deferred] makes it more reliable, especially when called inside [method Node._ready].
 			</description>
 		</method>
 		<method name="has_color" qualifiers="const">