[canvaskit] Add docs/types for remaining core functionality


Change-Id: Ib9f1f1eb942e83683af4c4d7b127de90b440c765
Bug: skia:10717
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/321938
Reviewed-by: Nathaniel Nifong <nifong@google.com>
2 files changed