spanner: documentation for update mutation does not include the requirement to include the PK value

The documentation for mutations does not explicitly state that an `Update` must include all the values of the primary key: https://github.com/googleapis/google-cloud-go/blob/2894e66be7ff7536f725ede453d1834586a361bd/spanner/mutation.go#L72

This can be slightly confusing, as for example delete mutations allow the use of key sets or key ranges to delete multiple rows with one mutation, while `Update` mutations can only operate on one row.