proposal: more updates to code coverage revamp design document
proposal: more updates to code coverage revamp design document

More specific details on user-callable APIs for writing coverage data.

Updates golang/go#51430.

Change-Id: I9ebe2f20ee7fc6b37aadef7fd16dc97ee5b67da8
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/409994
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/proposal,golang/proposal