man: Remove client attribute description

Remove attribute description from clients.conf(5) source as it is
outdated, lists just a few attributes and wasn't updated for more than a
year. Refer to clients.conf file itself, instead.
man: Remove references to naslist and clients

Remove references to non-existent naslist and clients manpages as they
are confusing.
Merge pull request #847 from spbnick/man_clients_update

clients(5) update