Update some of the dox code to GrDirectContext

If we're to remove GrContext all uses must go!

Change-Id: I487a973004c4f080fc5802128770b417d54628e2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301981
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
10 files changed