gossip: do not try to connect if we are not validators (#2786)

* gossip: do not issue a connection request if we are not a validator

* guide updates

* use all relevant authorities when issuing a request

* use AuthorityDiscoveryApi instead

* update comments to the status quo

polkadot/roadmap/implementers-guide/src/SUMMARY.md

@@ -61,6 +61,7 @@
     - [Candidate Validation](node/utility/candidate-validation.md)
     - [Provisioner](node/utility/provisioner.md)
     - [Network Bridge](node/utility/network-bridge.md)
+    - [Gossip Support](node/utility/gossip-support.md)
     - [Misbehavior Arbitration](node/utility/misbehavior-arbitration.md)
     - [Peer Set Manager](node/utility/peer-set-manager.md)
     - [Runtime API Requests](node/utility/runtime-api.md)

polkadot/roadmap/implementers-guide/src/node/utility/gossip-support.md

@@ -0,0 +1,11 @@
+# Gossip Support
+
+The Gossip Support Subsystem is responsible for keeping track of session changes
+and issuing a connection request to all validators in the next, current and a few past sessions
+if we are a validator in these sessions.
+The request will add all validators to a reserved PeerSet, meaning we will not reject a connection request
+from any validator in that set.
+
+Gossiping subsystems will be notified when a new peer connects or disconnects by network bridge.
+It is their responsibility to limit the amount of outgoing gossip messages.
+At the moment we enforce a cap of `max(sqrt(peers.len()), 25)` message recipients at a time in each gossiping subsystem.