Document occlusion debugging in the SDFGI debug probes draw mode in the editor

This feature shows occlusion information for the selected SDFGI
probe. It can only be used in the editor, as it relies
on the `RenderingServer::sdfgi_set_debug_probe_select()` method
that isn't exposed to the scripting API.

doc/classes/RenderingServer.xml

@@ -5333,6 +5333,7 @@
 		</constant>
 		<constant name="VIEWPORT_DEBUG_DRAW_SDFGI_PROBES" value="17" enum="ViewportDebugDraw">
 			Draws SDFGI probe data. This is the data structure that is used to give indirect lighting dynamic objects moving within the scene.
+			When in the editor, left-clicking a probe will display additional bright dots that show its occlusion information. A white dot means the light is not occluded at all at the dot's position, while a red dot means the light is fully occluded. Intermediate values are possible.
 			[b]Note:[/b] Only supported when using the Forward+ rendering method.
 		</constant>
 		<constant name="VIEWPORT_DEBUG_DRAW_GI_BUFFER" value="18" enum="ViewportDebugDraw">

doc/classes/Viewport.xml

@@ -712,6 +712,7 @@
 		</constant>
 		<constant name="DEBUG_DRAW_SDFGI_PROBES" value="17" enum="DebugDraw">
 			Draws the probes used for signed distance field global illumination (SDFGI).
+			When in the editor, left-clicking a probe will display additional bright dots that show its occlusion information. A white dot means the light is not occluded at all at the dot's position, while a red dot means the light is fully occluded. Intermediate values are possible.
 			Does nothing if the current environment's [member Environment.sdfgi_enabled] is [code]false[/code].
 			[b]Note:[/b] Only supported when using the Forward+ rendering method.
 		</constant>