document when we retired a serialized verb -- for later removal

BUG=skia:

Change-Id: I86a077080f350a900ad6605efb3b034204dd636a
Reviewed-on: https://skia-review.googlesource.com/9964
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed