Add best practice: stop_headsign does not "carry down"

[giocorti]

This is one of 5 PRs that incorporates a recommendation from the GTFS best practices.

This PR adds explains that stop_headsign does not carry down

The 4 related PRs can are:

• Headsign and route name examples
• Specifics about in-seat transfers
• stop_id is not rider facing
• Include arrival and departure times when possible

[CLAassistant]

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2 out of 3 committers have signed the CLA.

✅ antrim
✅ heidiguenin
❌ giocorti

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