doc change for getAlphaf

Please add this to your CL when you reland.
Thanks!

R=reed@google.com
NOTRY=true
Docs-Preview: https://skia.org/?cl=156007
Bug: skia:
Change-Id: Iba458272d34d2dee412b95baeabaf6eff181e93a
Reviewed-on: https://skia-review.googlesource.com/156007
Commit-Queue: Cary Clark <caryclark@google.com>
Reviewed-by: Mike Reed <reed@google.com>
4 files changed