Add a comment pointing to a related spec section.

This is a diagram about the order of operations of transforms; it seems
useful to add a pointer to the section of the spec that defines such an
order, even though the structure of the definitions is a bit different.

Change-Id: I6f3574ccd2ba3da66031a8b820551457fd14a8b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2773532


Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: L. David Baron <dbaron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864892}