update godoc

Author: davecheney

errors.go

@@ -49,9 +49,13 @@
 // invoked. This information can be retrieved with the following interface.
 //
 //     type Stacktrace interface {
-//             Stacktrace() []Frame
+//             Stacktrace() errors.Stacktrace
 //     }
 //
+// Where errors.Stacktrace is defined as
+//
+//     type Stacktrace []Frame
+//
 // The Frame type represents a call site in the stacktrace. Frame supports
 // the fmt.Formatter interface that can be used for printing information about
 // the stacktrace of this error. For example: