With 462505f310 "include refresh" most of
the useful parts of the SkFontMetrics documentation were removed.
Restore the documentation and update it with the latest information.
Change-Id: I28c13bc194b53265818e31e7b26e93a390f6259c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/234945
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>