add color4f to docs

Add SkPaint::setColor4f and getColor4f.

R=brianosman@google.com
NOTRY=true
Docs-Preview: https://skia.org/?cl=151761
Bug: skia:6898
Change-Id: If7966a4e98318a145870f3d17bd6dbc1b3cf9439
Reviewed-on: https://skia-review.googlesource.com/151761
Auto-Submit: Cary Clark <caryclark@skia.org>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed