fix a couple of minor surface doc changes

Newly added MakeRenderTarget doesn't have
an example yet, so just mark it as such;
and remove the doxygen style comments.

MakeFormBackendRenderTarget was missing

TBR=robertphillips@google.com

Docs-Preview: https://skia.org/?cl=117629
NOTRY=true
Bug: skia:6898
Change-Id: Icf6fb7195503b54c96cbf3a5785c2c4b3c0f9d8d
Reviewed-on: https://skia-review.googlesource.com/117629
Commit-Queue: Cary Clark <caryclark@skia.org>
Reviewed-by: Cary Clark <caryclark@skia.org>
2 files changed