Correct docs for Environment background keep mode

The docs incorrectly stated that KEEP was the fastest mode. This is not the case with modern hardware.

(cherry picked from commit bc924d4b4135e3314fbf934aca5274c7985a4408)

doc/classes/Environment.xml

@@ -302,7 +302,8 @@
 	</members>
 	<constants>
 		<constant name="BG_KEEP" value="5" enum="BGMode">
-			Keeps on screen every pixel drawn in the background. This is the fastest background mode, but it can only be safely used in fully-interior scenes (no visible sky or sky reflections). If enabled in a scene where the background is visible, "ghost trail" artifacts will be visible when moving the camera.
+			Keeps on screen every pixel drawn in the background. Only select this mode if you really need to keep the old data. On modern GPUs it will generally not be faster than clearing the background, and can be significantly slower, particularly on mobile.
+			It can only be safely used in fully-interior scenes (no visible sky or sky reflections). If enabled in a scene where the background is visible, "ghost trail" artifacts will be visible when moving the camera.
 		</constant>
 		<constant name="BG_CLEAR_COLOR" value="0" enum="BGMode">
 			Clears the background using the clear color defined in [member ProjectSettings.rendering/environment/default_clear_color].