wip pixmap docs

wip pixmap docs

Docs-Preview: https://skia.org/?cl=42522
Bug: skia: 6898
Change-Id: I85947bc36ea057ed008b87d7bef2efa82d7c89ad
Reviewed-on: https://skia-review.googlesource.com/42522
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
18 files changed