Add a comment explaining the buffer binding points. BUG=angleproject:881 Change-Id: If81675ef4931c920edac56249461db7c91cc170c Reviewed-on: https://chromium-review.googlesource.com/261708 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>