Remove getShapedRuns, add some dox

Change-Id: I7e226622475f7ee7173e7d2ec6fc388bd1792253
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/403598
Reviewed-by: Yegor Jbanov <yjbanov@google.com>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed