nixos: capitalize a bunch of service descriptions

(systemd service descriptions that is, not service descriptions in "man
configuration.nix".)

Capitalizing each word in the description seems to be the accepted
standard.

Also shorten these descriptions:
 * "Munin node, the agent process" => "Munin Node"
 * "Planet Venus, an awesome ‘river of news’ feed reader" => "Planet Venus Feed Reader"