DatagramSocket.dox: fixed ReceiveFrom() description.

* Please read other documentation and code before trying to write any
  documentation for Haiku. While any effort to improve our documentation
  is welcome, adding non-sense to it is harmful. Writing proper
  documentation is hard.
* Fixed spelling in a number of cases.
* Please don't write method names without (). Please use \a and \c to
  mark the text accordingly.