Cleanup redundant angle:: prefix in Features headers.
Cleanup workaround descriptions.

Bug: angleproject:1621
Change-Id: I4325d3603877efee9c6a035cd5b5bbbbc4f5c169
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1625609Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>