update docs around fonthinting

follow-on to https://skia-review.googlesource.com/c/skia/+/173432

Bug: skia:
Change-Id: I0cf30c30dc3aae9733b7f4a6830814c288647f42
Reviewed-on: https://skia-review.googlesource.com/c/173640
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Cary Clark <caryclark@google.com>
2 files changed