oauth2: mention RFC 6749 in package documentation

Change-Id: Iccf5c53fd5ea3c98447b6c6f4b751655847f1b5e
Reviewed-on: https://go-review.googlesource.com/115936
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/oauth2.go b/oauth2.go
index a047a5f..10299d2 100644
--- a/oauth2.go
+++ b/oauth2.go
@@ -3,7 +3,8 @@
 // license that can be found in the LICENSE file.
 // Package oauth2 provides support for making
-// OAuth2 authorized and authenticated HTTP requests.
+// OAuth2 authorized and authenticated HTTP requests,
+// as specified in RFC 6749.
 // It can additionally grant authorization with Bearer JWT.
 package oauth2 // import "golang.org/x/oauth2"