Document SkTextBlobBuilder::allocRunText*
Document SkTextBlobBuilder::allocRunText*

Now that these methods are used in Chromium to produce PDFs with
embedded text, properly document how to use them.

Change-Id: I68fb477b65ec41af9fcc46429275bda03680bff0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/387976
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Reviewed-by: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>
Reviewed-by: Dominik Röttsches <4af871057802bf79ede29fabac3fc1660707b3d3@chromium.org>

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