doc/conventions: sub, hook

doc/conventions.txt

@@ -41,6 +41,10 @@ Naming:
 - "type": Numeric id representing a type
 - "name": A string; generally works as an identifier but not necessarily
 - "id":   Numeric id of an instance of something that is not a type
+- "sub":  Subscription; a means to trigger something based on something else
+          happening; data goes from the emitter to the subscriber
+- "hook": A callback that can modify some of the source's data or behavior
+          before the source goes on with it
 
 Logging:
 - Use core/log.h. Only use stdout directly in case of an interactive command