Booking form: Add helper text to callsign and aircraft

[daveroverts]

Even though a Vatsim member should already know this, they still use IATA codes for callsign and/or aircraft codes. This only applies to a booking where these 2 fields are editable.

I think a helper text like the one below could help

[github-actions]

🎉 This issue has been resolved in version 4.2.0-beta.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[github-actions]

🎉 This issue has been resolved in version 5.0.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀