Update documentation for ServiceUpdate

Currently, the behaviour for the version field in ServiceUpdate()
is vague. Without an correct version number, users are unable to
successfully run ServiceUpdate(), which is a pretty critical method
for scaling services (for example). I've just added an extra sentence
explaining what the version number is for, and where to find it.

Signed-off-by: Harrison Turton <harrisonturton@gmail.com>

client/service_update.go

@@ -10,7 +10,9 @@ import (
 	"github.com/docker/docker/api/types/swarm"
 )
 
-// ServiceUpdate updates a Service.
+// ServiceUpdate updates a Service. The version number is required to avoid conflicting writes.
+// It should be the value as set *before* the update. You can find this value in the Meta field
+// of swarm.Service, which can be found using ServiceInspectWithRaw.
 func (cli *Client) ServiceUpdate(ctx context.Context, serviceID string, version swarm.Version, service swarm.ServiceSpec, options types.ServiceUpdateOptions) (types.ServiceUpdateResponse, error) {
 	var (
 		query   = url.Values{}