Document SkShaper::RunHandler::Buffer
Document SkShaper::RunHandler::Buffer

This at least documents the current behavior. There exists tension between
users who just want positions and those who also want advances, which
requires this to be overly clever. Perhaps a better solution can be
found.

Change-Id: Ic2166ca294003da3325a0fe068ef84cdef9f804d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/217259
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Commit-Queue: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>
Reviewed-by: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>

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