Document default values for vocabulary-specific configuration settings in the wiki

[miguelvaara]

Follow the way presented in the Global configuration section (Default value if not set) on the page https://github.com/NatLibFi/Skosmos/wiki/Configuration

[osma]

@miguelvaara has been adding default values to the table. I reviewed them and made some changes directly into the wiki, because there is no PR mechanism for wiki edits.

I changed a lot of null values into (unset) or something more appropriate for the context of Turtle configuration, where null is not meaningful.

I think we are done with this, so closing the issue. Please reopen if necessary - or for minor tweaks, just edit the wiki page directly.