tweak SkImage documentation

- fix syntactic nits
- add how to check result of getBackendTexture
- add example

R=robertphillips@google.com
TBR=reed@google.com
Docs-Preview: https://skia.org/?cl=118590
Bug: skia:6898
Change-Id: I60cfcf448dc2f201ecc34d33ead707d0c3e3b86b
Reviewed-on: https://skia-review.googlesource.com/118590
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Cary Clark <caryclark@skia.org>
6 files changed