Update shader translator docs. Bug: angleproject:528 Change-Id: I73cea8b2127abfe6ffa363f7abaabe039290c680 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3199995 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>