LLMNR and mDNS settings can have their global default value configured
in "NetworkManager.conf".
Global default values should work the way that all regular values of the property
can be configured explicitly in the connection profile. The special "default" value
only indicates to allow lookup of the global default, but it should not have a
meaning of its own.
Note that if mDNS/LLMNR settings are left unspecified, we will set the
argument to SetLinkMulticastDNS() and SetLinkLLMNR() functions to "",
which means that systemd-resolved decides on a default. Also, depending
on the DNS plugin, the default value differs. This is all fine however.
In this case, the ultimate default value depends on other things (like
the DNS plugin), but each possible value is in fact explicitly
configurable. We also do that for "ipv6.ip6-privacy".
Anyway, cleanup the documentation a bit and try to better explain what
the default is.
3d07708f59