The underline position is actually the top of the underline. Also
clarify that the strike through position is the bottom of the strike
through.
This adds documentation that the values are always y-down in a way that
will end up in the SkPaint header file for those who look for
information there.
Change-Id: I40281c0e47557d23291b6a91474e2d3e3b37f09f
Docs-Preview: https://skia.org/user/api/SkPaint_Reference?cl=69361#FontMetrics
Reviewed-on: https://skia-review.googlesource.com/69361
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
e580649a38