Document that tangents are required for normal mapping in SpatialMaterial

doc/classes/SpatialMaterial.xml

@@ -230,7 +230,8 @@
 			The strength of the normal map's effect.
 		</member>
 		<member name="normal_texture" type="Texture" setter="set_texture" getter="get_texture">
-			Texture used to specify the normal at a given pixel. The [code]normal_texture[/code] only uses the red and green channels. The normal read from [code]normal_texture[/code] is oriented around the surface normal provided by the [Mesh].
+			Texture used to specify the normal at a given pixel. The [code]normal_texture[/code] only uses the red and green channels; the blue and alpha channels are ignored. The normal read from [code]normal_texture[/code] is oriented around the surface normal provided by the [Mesh].
+			[b]Note:[/b] The mesh must have both normals and tangents defined in its vertex data. Otherwise, the normal map won't render correctly and will only appear to darken the whole surface. If creating geometry with [SurfaceTool], you can use [method SurfaceTool.generate_normals] and [method SurfaceTool.generate_tangents] to automatically generate normals and tangents respectively.
 			[b]Note:[/b] Godot expects the normal map to use X+, Y-, and Z+ coordinates. See [url=http://wiki.polycount.com/wiki/Normal_Map_Technical_Details#Common_Swizzle_Coordinates]this page[/url] for a comparison of normal map coordinates expected by popular engines.
 		</member>
 		<member name="params_alpha_scissor_threshold" type="float" setter="set_alpha_scissor_threshold" getter="get_alpha_scissor_threshold">