Apply xbean documentation recommendations to the servicemix-mail component

Apply the recommendations outlined in http://cwiki.apache.org/confluence/display/SMX4/Annotating+servicemix+endpoints+for+auto-generated+documentation to the servicemix-mail component to facilitate auto generation of quality comments for XSD, Wiki, HTML & JavaDoc.

Right now the properties are only very loosely documented: we should document each one explicitly, including stating the default value (if one applies), aiming the documentation at developers who can use the docu for context sensitive help in Eclipse.

Also, would be good to include descriptions of the connection Uris, as per the documentation at http://servicemix.apache.org/servicemix-mail.html.