Created a macro to help keep Features' descriptions up to date. This
will avoid confusion in the future when conditions change. Also update
all descriptions to match current state.

Bug: angleproject:3947
Change-Id: Ifc65e7789c916fab79f1323798dfb59d7a4efad2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1829584
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>