Add Stretch Mode description to ProjectSettings.xml

Update doc/classes/ProjectSettings.xml

Co-authored-by: Hugo Locurcio <[email protected]>
(cherry picked from commit 77a61084aa6f083a81bab0f56df1a413feb5a08b)

doc/classes/ProjectSettings.xml

@@ -710,6 +710,10 @@
 		<member name="display/window/stretch/aspect" type="String" setter="" getter="" default="&quot;keep&quot;">
 		</member>
 		<member name="display/window/stretch/mode" type="String" setter="" getter="" default="&quot;disabled&quot;">
+			Defines how the base size is stretched to fit the resolution of the window or screen.
+			[b]"disabled"[/b]: No stretching happens. One unit in the scene corresponds to one pixel on the screen. In this mode, [member display/window/stretch/aspect] has no effect. Recommended for non-game applications.
+			[b]"canvas_items"[/b]: The base size specified in width and height in the project settings is stretched to cover the whole screen (taking [member display/window/stretch/aspect] into account). This means that everything is rendered directly at the target resolution. 3D is unaffected, while in 2D, there is no longer a 1:1 correspondence between sprite pixels and screen pixels, which may result in scaling artifacts. Recommended for most games that don't use a pixel art aesthetic, although it is possible to use this stretch mode for pixel art games too (especially in 3D).
+			[b]"viewport"[/b]: The size of the root [Viewport] is set precisely to the base size specified in the Project Settings' Display section. The scene is rendered to this viewport first. Finally, this viewport is scaled to fit the screen (taking [member display/window/stretch/aspect] into account). Recommended for games that use a pixel art aesthetic.
 		</member>
 		<member name="display/window/stretch/scale" type="float" setter="" getter="" default="1.0">
 		</member>