sync docs up with tip of tree

Also check in work in progress for blend modes,
round rects, and a placeholder for pictures.

One minor include change to add a parameter name
for SkBlendMode function.

TBR=reed@google.com
R=caryclark@google.com

Docs-Preview: https://skia.org/?cl=134200
Bug: skia:6898
Change-Id: I5d2a9221d61edb32d9c7edbb3193401605b2b513
Reviewed-on: https://skia-review.googlesource.com/134200
Reviewed-by: Cary Clark <caryclark@google.com>
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>
39 files changed