Fix docs for commands accepting specs

[kontura]

install, upgrade and remove commands are documented to accept only packages (<package-spec>) but they work also with comps.

[ppisar]

It's not only documentation. "dnf install --help" also reads:

Arguments:                                     
  specs                                        List of package specs to install

I think package-spec identifier should be kept reserved for packages and would should change documentation to package-spec|group-spec where appropriate. Otherwise, the grammar would accept @group > 2 which is obviously unwanted.