Document how to avoid light leaks with GIProbe

doc/classes/GIProbe.xml

@@ -6,6 +6,7 @@
 	<description>
 		[GIProbe]s are used to provide high-quality real-time indirect light to scenes. They precompute the effect of objects that emit light and the effect of static geometry to simulate the behavior of complex light in real-time. [GIProbe]s need to be baked before using, however, once baked, dynamic objects will receive light from them. Further, lights can be fully dynamic or baked.
 		Having [GIProbe]s in a scene can be expensive, the quality of the probe can be turned down in exchange for better performance in the [ProjectSettings] using [member ProjectSettings.rendering/quality/voxel_cone_tracing/high_quality].
+		[b]Note:[/b] Meshes should have sufficiently thick walls to avoid light leaks (avoid one-sided walls). For interior levels, enclose your level geometry in a sufficiently large box and bridge the loops to close the mesh.
 		[b]Note:[/b] Due to a renderer limitation, emissive [ShaderMaterial]s cannot emit light when used in a [GIProbe]. Only emissive [SpatialMaterial]s can emit light in a [GIProbe].
 	</description>
 	<tutorials>