Document default values for all settings

[provokateurin]

I started configuring my server and setting lots of config values, but also started reading the config parsing code. I found out, that often the default value was exactly what I was setting manually (to increase security; good job on using secure defaults everywhere!), because the documentation almost never states what the default value for a config key is. It would be much easier, if those were properly documented, so one could see that the defaults are already very good.