docs: update references to NetworkConfiguration::extra_sets (#7386)

* docs: update references to NetworkConfiguration::extra_sets

Since paritytech/substrate#14080, this struct field no longer exists,
now the `add_notification_protocol()` function of
`sc_network::config::FullNetworkConfiguration` is used.

Also neuter the doc links for now; rustdoc can't resolve them
(presumably because sc_network::config isn't in scope, though weirdly
enough even spelling the link out as
``[`FullNetworkConfiguration`](struct@sc_network::config::FullNetworkConfiguration)``
doesn't work?). Normally this wouldn't be an issue and rustdoc would
just not generate links, but rust 1.70 has a bug that completely crashes
rustdoc in this case.