Also, the isindex element has been remove from specification,
so this commit adds a comment about it.

Change-Id: I79a9b1eb9dae8274e2ca498ab73b2e73521d54e9
Reviewed-on: https://go-review.googlesource.com/64230Reviewed-by: Tom Bergan <tombergan@google.com>
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>