Update Texture dirty bits documentation. BUG=angleproject:1387 Change-Id: Ie2277874acb9e7a3eed4bce327f2c08750213967 Reviewed-on: https://chromium-review.googlesource.com/660419 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>