Document visibility AABB caveats with decals, reflection probes and lights

This is required in rendering methods that don't use clustering
to check which meshes should be affected by decals, reflection probes
and lights.

doc/classes/Decal.xml

@@ -8,6 +8,7 @@
 		They are made of an [AABB] and a group of [Texture2D]s specifying [Color], normal, ORM (ambient occlusion, roughness, metallic), and emission. Decals are projected within their [AABB] so altering the orientation of the Decal affects the direction in which they are projected. By default, Decals are projected down (i.e. from positive Y to negative Y).
 		The [Texture2D]s associated with the Decal are automatically stored in a texture atlas which is used for drawing the decals so all decals can be drawn at once. Godot uses clustered decals, meaning they are stored in cluster data and drawn when the mesh is drawn, they are not drawn as a post-processing effect after.
 		[b]Note:[/b] Decals cannot affect an underlying material's transparency, regardless of its transparency mode (alpha blend, alpha scissor, alpha hash, opaque pre-pass). This means translucent or transparent areas of a material will remain translucent or transparent even if an opaque decal is applied on them.
+		[b]Note:[/b] When using the Mobile rendering method, decals will only correctly affect meshes whose visibility AABB intersects with the decal's AABB. If using a shader to deform the mesh in a way that makes it go outside its AABB, [member GeometryInstance3D.extra_cull_margin] must be increased on the mesh. Otherwise, the decal may not be visible on the mesh.
 	</description>
 	<tutorials>
 	</tutorials>