common/mutation.go: godoc comments for API.

Renamed GetKeyVersion to GetKeyVersions

Change-Id: Id98c0fd461e2e8c2cc72d2a1083dffe5dcb330ae
Reviewed-on: http://review.couchbase.org/35674
Reviewed-by: Manik Taneja <manik@couchbase.com>
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>