Tui: warn about large flows

[oliver-sanders]

Address a minor point on the beta-0 roadmap.

Add a warning about using with large flows until we get it delta-driven.

See #3527

One off-topic change, got shellchecker to skip the .git dir.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• One off-topic change listed above
• Does not need tests (why?).
• No change log entry required (why? e.g. invisible to users).
• No documentation update required.
• No dependency changes.

[hjoliver]

I think we can forgive the codecov patch fail in this case.