Format and comment SkScalarInterpFunc

Consistently use less than for comparisons and ordering and add braces.
Make documentation a bit more direct.

Change-Id: I3286ff3fac3e276b2e40afab5c9a27e202eab9cf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/313556
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
2 files changed