iroh-net: default features are not compatible with doc tests

[dignifiedquire]

$ cargo test -p iroh-net

fails with

---- iroh-net/src/discovery.rs - discovery (line 73) stdout ----
error[E0432]: unresolved import `iroh_net::discovery::local_swarm_discovery`
   --> iroh-net/src/discovery.rs:76:26
    |
6   | use iroh_net::discovery::local_swarm_discovery::LocalSwarmDiscovery;
    |                          ^^^^^^^^^^^^^^^^^^^^^ could not find `local_swarm_discovery` in `discovery`
    |
note: found an item that was configured out
   --> /Users/dignified/opensource/iroh/iroh-net/src/discovery.rs:115:9
    |
115 | pub mod local_swarm_discovery;
    |         ^^^^^^^^^^^^^^^^^^^^^
note: the item is gated behind the `discovery-local-network` feature
   --> /Users/dignified/opensource/iroh/iroh-net/src/discovery.rs:113:7
    |
113 | #[cfg(feature = "discovery-local-network")]
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0432`.
Couldn't compile the test.

failures:
    iroh-net/src/discovery.rs - discovery (line 73)