add simple docs for textencoding in SkPaint

Bug: skia:
Change-Id: Id7cd721dc3d3e6297b2653c26cc35eca8dba0ec8
Reviewed-on: https://skia-review.googlesource.com/c/174591
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
3 files changed