feat: add note to 0011_multiple_event_types_per_topic.rst

[robrap]

Add additional note on how consumers find the schema.

Merge checklist: Check off if complete or not applicable:

• [ ] Version bumped
• [ ] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [ ] Commits are squashed
• [ ] Noted any: Concerns, dependencies, deadlines, tickets, testing instructions