Clarify what UV means in CanvasItem shaders (#3933)

tutorials/shading/shading_reference/canvas_item_shader.rst

@@ -125,7 +125,7 @@ is usually:
 |                                | For a Sprite with a texture of size 64x32px,                   |
 |                                | **TEXTURE_PIXEL_SIZE** = :code:`vec2(1/64, 1/32)`              |
 +--------------------------------+----------------------------------------------------------------+
-| inout vec2 **UV**              | UV.                                                            |
+| inout vec2 **UV**              | Texture coordinates.                                           |
 +--------------------------------+----------------------------------------------------------------+
 | inout vec4 **COLOR**           | Color from vertex primitive.                                   |
 +--------------------------------+----------------------------------------------------------------+