docs: Add a document explaining email/push notifications.

With various fixes by Mateusz Mandera.

docs/subsystems/events-system.md

@@ -161,6 +161,9 @@ Server (which stores queues in memory) were to crash and lose
 its data, clients would recover, just as if they had lost Internet
 access briefly (there is some DoS risk to manage, though).
 
+Note that the garbage-collection system has hooks that are important
+for the implementation of [notifications](../subsystems/notifications.md).
+
 (The Event Queue Server is designed to save any event queues to disk
 and reload them when the server is restarted, and catches exceptions
 carefully, so such incidents are very rare, but it's nice to have a