Document how to avoid light leaks in GI Probes (#4784)

tutorials/3d/gi_probes.rst

@@ -50,6 +50,12 @@ toolbar to begin the pre-baking process:
 
 .. image:: img/giprobe_bake.png
 
+.. warning::
+
+    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.
+
 Adding lights
 -------------