update docs for MakeFromString and SkTextEncoding
update docs for MakeFromString and SkTextEncoding

Bug: skia:
Change-Id: I12d7c73278490889e7829335ea61c98ddbde4f9f
Reviewed-on: https://skia-review.googlesource.com/c/173989
Reviewed-by: Cary Clark <52792aa984ff6c4e9c802c7fa3093ed6a52a791f@google.com>
Commit-Queue: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>

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