add comments about the use of different subrun types

Change-Id: I117485a784d7ceec5b89d73d08647a89a1a6be7d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330225
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
1 file changed