Update developer stream docs to point out _setup_sequence(...) in portdb

[MadLittleMods]

Update developer stream docs to point out _setup_sequence(...) in portdb

Part of #19671

Spawning from discussion in #synapse-dev:matrix.org with roots in #19558 (comment). As trialed/discovered by @turt2live alongside @reivilibre and @clokep ❤️

Why is this necessary?

If you forget to add _setup_sequence(...), you can run into the following error if there is 1 row in SQLite and then you use the portdb script to try to migrate to Postgres (as explained by @reivilibre)

Postgres sequence 'quarantined_media_id_seq' is inconsistent with associated stream position
of 'quarantined_media' in the 'stream_positions' table.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[reivilibre]

Thanks!

[MadLittleMods]

Thanks for the review @reivilibre 🦛