oauth2: remove misleading Transport.RoundTrip comment

This removes any assumption about how the underlying TokenSource is

Change-Id: I03521e2f26bd07e25eb6ba00a78c285ece1835cc
Reviewed-on: https://go-review.googlesource.com/114955
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/transport.go b/transport.go
index c55bfa0..aa0d34f 100644
--- a/transport.go
+++ b/transport.go
@@ -31,8 +31,7 @@
 // RoundTrip authorizes and authenticates the request with an
-// access token. If no token exists or token is expired,
-// tries to refresh/fetch a new token.
+// access token from Transport's Source.
 func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
 	reqBodyClosed := false
 	if req.Body != nil {