Beef up the comment in the file to describe what's going on with
the licenses, as well as describe why the generated file is
a different license.
Also move the BSD text to a different file.
Change-Id: I6d08256317f3a0a150e0c35d3d902fceb7dcb44e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/72680
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>
Shrek Shao