Improve dget help and man page to clarify difference to modern apt-get

The man page of dget had incorrect references to 'apt-get' (probably due historical reasons before apt-get got improved) and it was unclear what new Debian Developers are supposed to use dget for. Clarify the main use case and add examples to make the utility of dget clean, and also update the 'apt-get' reference to list the one repository download case apt-get does not yet handle (get all binaries for source package name).

Related: Bug#1059551