Remove GrBackendRenderTargetDesc in favor of GrBackendRenderTarget.

Also removes a reference to GrBackendTextureDesc in a comment and updates markdown docs.

Docs-Preview: https://skia.org/?cl=24861
Bug: skia:
Change-Id: Ic6490d5ef46953450e6dee69271397bb2b94d0d6
Reviewed-on: https://skia-review.googlesource.com/24861
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
12 files changed