firestore: fix serverTimestamp docs
Add into serverTimestamp comment tip about that it's non-zero values
are not affecting anything.
Fixes
@@ -122,9 +122,11 @@ func (d *DocumentRef) Get(ctx context.Context) (_ *DocumentSnapshot, err error)
//
// - omitempty: Do not encode this field if it is empty. A value is empty
// if it is a zero value, or an array, slice or map of length zero.
-// - serverTimestamp: The field must be of type time.Time. When writing, if
-// the field has the zero value, the server will populate the stored document with
-// the time that the request is processed.
+// - serverTimestamp: The field must be of type time.Time. serverTimestamp
+// is a sentinel token that tells Firestore to substitute the server time
+// into that field. When writing, if the field has the zero value, the
+// server will populate the stored document with the time that the request
+// is processed. However, if the field value is non-zero it won't be saved.
func (d *DocumentRef) Create(ctx context.Context, data interface{}) (_ *WriteResult, err error) {
ctx = trace.StartSpan(ctx, "cloud.google.com/go/firestore.DocumentRef.Create")
defer func() { trace.EndSpan(ctx, err) }()
