Improve BoxMesh documentation about UV scaling for vertex shader

Added comment that translates the UV scaling from the standard shader properties
to how that's being done in the godot vertex shader.

doc/classes/BoxMesh.xml

@@ -5,7 +5,7 @@
 	</brief_description>
 	<description>
 		Generate an axis-aligned box [PrimitiveMesh].
-		The box's UV layout is arranged in a 3×2 layout that allows texturing each face individually. To apply the same texture on all faces, change the material's UV property to [code]Vector3(3, 2, 1)[/code].
+		The box's UV layout is arranged in a 3×2 layout that allows texturing each face individually. To apply the same texture on all faces, change the material's UV property to [code]Vector3(3, 2, 1)[/code]. This is equivalent to adding [code]UV *= vec2(3.0, 2.0)[/code] in a vertex shader.
 		[b]Note:[/b] When using a large textured [BoxMesh] (e.g. as a floor), you may stumble upon UV jittering issues depending on the camera angle. To solve this, increase [member subdivide_depth], [member subdivide_height] and [member subdivide_width] until you no longer notice UV jittering.
 	</description>
 	<tutorials>