update docs for MakeFromString and SkTextEncoding

Bug: skia:
Change-Id: I12d7c73278490889e7829335ea61c98ddbde4f9f
Reviewed-on: https://skia-review.googlesource.com/c/173989
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed