The transition from mark 1 to mark 2 no longer enqueues new root
marking jobs, but some of the comments still refer to this. Fix these
comments.

Change-Id: I3f98628dba32c5afe30495ab495da42b32291e9e
Reviewed-on: https://go-review.googlesource.com/24965Reviewed-by: Rick Hudson <rlh@golang.org>