working on SkImage docs

also fix minor break in SkSurface

TBR=caryclark@google.com
Docs-Preview: https://skia.org/?cl=105021
Bug: skia:
Change-Id: I0cfc01ab5ba4df13a9e84f8dd2904d32e5726a5b
Reviewed-on: https://skia-review.googlesource.com/105021
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
11 files changed