proto: adjust documentation on RequiredNotSetError (#603)

The new table-drive unmarshaler never emits "{Unknown}",
so that comment is stale.

Also, there is no reason to document that the name of "first" missing
required field is stored:
* There is no programmatic way to access this field, so it is just for user debugging.
* "First" is not well defined. First encounted while parsing? First listed in proto file? First listed in the Go struct?

proto/encode.go

@@ -41,20 +41,17 @@ import (
 	"reflect"
 )
 
-// RequiredNotSetError is the error returned if Marshal is called with
-// a protocol buffer struct whose required fields have not
-// all been initialized. It is also the error returned if Unmarshal is
-// called with an encoded protocol buffer that does not include all the
-// required fields.
-//
-// When printed, RequiredNotSetError reports the first unset required field in a
-// message. If the field cannot be precisely determined, it is reported as
-// "{Unknown}".
+// RequiredNotSetError is an error type returned by either Marshal or Unmarshal.
+// Marshal reports this when a required field is not initialized.
+// Unmarshal reports this when a required field is missing from the wire data.
 type RequiredNotSetError struct {
 	field string
 }
 
 func (e *RequiredNotSetError) Error() string {
+	if e.field == "" {
+		return fmt.Sprintf("proto: required field not set")
+	}
 	return fmt.Sprintf("proto: required field %q not set", e.field)
 }