Clarify `NOTIFICATION_SCROLL_BEGIN/END` behavior

Documents that these notifications are only sent for touch events.

(cherry picked from commit 44f62a4f97881918182db55a6c3e1c259ed14f7b)

doc/classes/Control.xml

@@ -1161,10 +1161,12 @@
 			[b]Note:[/b] As an optimization, this notification won't be sent from changes that occur while this node is outside of the scene tree. Instead, all of the theme item updates can be applied at once when the node enters the scene tree.
 		</constant>
 		<constant name="NOTIFICATION_SCROLL_BEGIN" value="47">
-			Sent when this node is inside a [ScrollContainer] which has begun being scrolled.
+			Sent when this node is inside a [ScrollContainer] which has begun being scrolled when dragging the scrollable area [i]with a touch event[/i]. This notification is [i]not[/i] sent when scrolling by dragging the scrollbar, scrolling with the mouse wheel or scrolling with keyboard/gamepad events.
+			[b]Note:[/b] This signal is only emitted on Android or iOS, or on desktop/web platforms when [member ProjectSettings.input_devices/pointing/emulate_touch_from_mouse] is enabled.
 		</constant>
 		<constant name="NOTIFICATION_SCROLL_END" value="48">
-			Sent when this node is inside a [ScrollContainer] which has stopped being scrolled.
+			Sent when this node is inside a [ScrollContainer] which has stopped being scrolled when dragging the scrollable area [i]with a touch event[/i]. This notification is [i]not[/i] sent when scrolling by dragging the scrollbar, scrolling with the mouse wheel or scrolling with keyboard/gamepad events.
+			[b]Note:[/b] This signal is only emitted on Android or iOS, or on desktop/web platforms when [member ProjectSettings.input_devices/pointing/emulate_touch_from_mouse] is enabled.
 		</constant>
 		<constant name="NOTIFICATION_LAYOUT_DIRECTION_CHANGED" value="49">
 			Sent when control layout direction is changed.