polish picture and rrect docs

TBR=caryclark@google.com

Docs-Preview: https://skia.org/?cl=141263
Bug: skia:6818
Change-Id: I33a197892b3dd5788499dda9dd9e95ddab861388
Reviewed-on: https://skia-review.googlesource.com/141263
Commit-Queue: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>
Reviewed-by: Cary Clark <caryclark@skia.org>
13 files changed