Correct documentation.

configTransport() does not accespt closures anymore. None of the
registry based config() methods do either. This was left over by
accident when closure factories were removed.

Refs #3454

src/Network/Email/Email.php

@@ -1124,8 +1124,8 @@ class Email {
  *
  * @param string|array $key The configuration name to read/write. Or
  *   an array of multiple transports to set.
- * @param array|\Closure|AbstractTransport Either an array of configuration
- *   data, a Closure factory function, or a transport instance.
+ * @param array|AbstractTransport Either an array of configuration
+ *   data, or a transport instance.
  * @return mixed Either null when setting or an array of data when reading.
  * @throws \Cake\Error\Exception When modifying an existing configuration.
  */