Changes made: Updated the description for the <pubkey> variable within the context of an event with the 'e' tag. The updated comment explains that the `<pubkey>` SHOULD be the pubkey of the author of the e tagged event, and it is used in the outbox model to search for that event from the author's write relays where relay hints did not resolve the event.
I think that adding a mention marker would eliminate ambiguity for clients supporting both the deprecated and preferred conventions. I also think that this would allow for extensibility in adding new types of event mentions (for example if we want to add context for a note).