fix SkPath documentation

SkPath now returns reference to itself.
There's also a new addPoly() variant.
Update the docs to match.

TBR=reed@google.com,kjlubick@google.com

Docs-Preview: https://skia.org/?cl=147216
Bug: skia:6818
Change-Id: I0e40e942cc46a8994285dfea4c81e2042a4e689c
Reviewed-on: https://skia-review.googlesource.com/147216
Commit-Queue: Cary Clark <caryclark@skia.org>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Auto-Submit: Cary Clark <caryclark@skia.org>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Cary Clark <caryclark@skia.org>
5 files changed