Document UserDataProcessor data sent when no user

This paragraph was removed in d4ef9fa since it was no longer true
since 24bbda1, but then became true again in 819667c.

Author: tremby

src/Processors/UserDataProcessor.php

@@ -19,6 +19,10 @@
  * By default only the 'id' field from the user is attached; pass null or an
  * empty array as the 'only' option to override this, or configure otherwise as
  * you see fit.
+ *
+ * If no user is logged in, the data `['id' => null]` is still attached to the
+ * error object, to differentiate a case where no data on the current user is
+ * available from a case where it is known that no user is logged in.
  */
 class UserDataProcessor
 {