Prohibit inconsistent size state for SubViewport

Prohibit size changes of SubViewports with parent SubViewportContainers that have stretch mode enabled.

doc/classes/SubViewport.xml

@@ -26,6 +26,7 @@
 		</member>
 		<member name="size" type="Vector2i" setter="set_size" getter="get_size" default="Vector2i(512, 512)">
 			The width and height of the sub-viewport. Must be set to a value greater than or equal to 2 pixels on both dimensions. Otherwise, nothing will be displayed.
+			[b]Note:[/b] If the parent node is a [SubViewportContainer] and its [member SubViewportContainer.stretch] is [code]true[/code], the viewport size cannot be changed manually.
 		</member>
 		<member name="size_2d_override" type="Vector2i" setter="set_size_2d_override" getter="get_size_2d_override" default="Vector2i(0, 0)">
 			The 2D size override of the sub-viewport. If either the width or height is [code]0[/code], the override is disabled.

doc/classes/SubViewportContainer.xml

@@ -13,6 +13,7 @@
 	<members>
 		<member name="stretch" type="bool" setter="set_stretch" getter="is_stretch_enabled" default="false">
 			If [code]true[/code], the sub-viewport will be automatically resized to the control's size.
+			[b]Note:[/b] If [code]true[/code], this will prohibit changing [member SubViewport.size] of its children manually.
 		</member>
 		<member name="stretch_shrink" type="int" setter="set_stretch_shrink" getter="get_stretch_shrink" default="1">
 			Divides the sub-viewport's effective resolution by this value while preserving its scale. This can be used to speed up rendering.