services.nfs.settings is added for options that go into /etc/nfs.conf.
There are services.nfs.server.extraNfsdConfig and
services.nfs.extraConfig, but they have drawbacks. They overlap in
scope (nfs.extraConfig can also add nfsd options). They require that one
writes INI syntax. They often produce nfs.conf files with duplicate
section names, which is confusing.
This deprecates services.nfs.server.extraNfsdConfig and
services.nfs.extraConfig.
services.nfs.settings cannot be used together with
services.nfs.server.extraNfsdConfig or services.nfs.extraConfig.