Clarify NumCoordinateAxes

Change-Id: Ie8a4e31c7f0440e3285827d0a592427a8be3ba0d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51804
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: David Neto <dneto@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/ast/texture.h

@@ -47,11 +47,16 @@ std::ostream& operator<<(std::ostream& out, TextureDimension dim);
 /// @return true if the given TextureDimension is an array texture
 bool IsTextureArray(TextureDimension dim);
 
-/// Returns the number of axes in the coordinate for a dimensionality.
+/// Returns the number of axes in the coordinate used for accessing
+/// the texture, where an access is one of: sampling, fetching, load,
+/// or store.
 ///  None -> 0
 ///  1D -> 1
 ///  2D, 2DArray -> 2
 ///  3D, Cube, CubeArray -> 3
+/// Note: To sample a cube texture, the coordinate has 3 dimensions,
+/// but textureDimensions on a cube or cube array returns a 2-element
+/// size, representing the (x,y) size of each cube face, in texels.
 /// @param dim the TextureDimension to query
 /// @return number of dimensions in a coordinate for the dimensionality
 int NumCoordinateAxes(TextureDimension dim);