minor SkFont interface edits

Add missing comments.
Fix copy/pasted comments.
Change operator== to a const function.

R=reed@google.com

Docs-Preview: https://skia.org/?cl=172866
Bug: skia:
Change-Id: I4b89204ae674f93bfaf9d86a5d09a84cc8021146
Reviewed-on: https://skia-review.googlesource.com/c/172866
Reviewed-by: Mike Reed <reed@google.com>
Auto-Submit: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
2 files changed