Clarify comments in SkEdge

While dealing with a bug in this code, I came back to some comments
I left unfinished and filled out the explanations a bit.

Change-Id: I6ef6cc29a8c3cdee61bec919e12605d1626a96a2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1283276
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>
Reviewed-by: Alexis Cruz-Ayala <alexisdavidc@google.com>
5 files changed