Bug#1038946: please add a long description

[Tomas Pospisek]

Instead of solely having a short description that says "$FOO is a $BAZ for 
$BAR" where it's not immediately clear without particular knowledge what 
$FOO and $BAR are, please add a short text, that explains what $FOO and 
$BAR actually do. Something like:

"Xapp is an application that does XXXX. xdg-desktop-portal allows you
 to do this and that. This package adds infrastructure to support Xapp via
 xdg-desktop-portal on Cinnamon, MATE and Xfce."
Thanks,
*t