Enhancements:

 * The MessageSend dialplan application now takes an optional
   third argument that can set the message's "To" field on
   outgoing messages.  It's an alternative to using the
   MESSAGE(to) dialplan function.

   NOTE: No channel driver currently implements this field.  A
   follow-on commit for res_pjsip_messaging will implement it for
   the chan_pjsip channel driver.

 * To prevent confusion with the first argument, currently named
   "to", it's been renamed to "destination". Its function,
   creating the request URI, hasn't changed.

 * The documentation for MessageSend was updated to be
   more clear about the parameters and how they interact
   the MESSAGE() dialplan function.

 * With the rename of MessageSend's first parameter, and the fact
   that message.c references <info> elements in chan_sip.c,
   res_pjsip_messaging.c and res_xmpp, they each needed
   documentation updates to use MessageDestinationInfo instead of
   MessageToInfo.

 * appdocsxml.dtd was updated to include a missing element
   declaration for "dataType".  This was showing up as an error
   in Eclipse's dtd editor.

 * Despite the changes in this commit, there should be
   no impact to current users of MessageSend.

Change-Id: I6fb5b569657a02866a66ea352fd53d30d8ac965a