check-help: visual and content improvements

check-help: visual and content improvements

The "check" target is not a target that will run all other tests
listed, so in order to be accurate it's necessary to list those that
will run.  The same is true for "check-clean".

Then, to give a better visual impression of the differences in the
various targets, let's add empty lines.

Finally, a small (and hopeful) grammar fix from a non-native speaker.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20181109150710.31085-5-crosa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>