Revise fallback.md

[jlebar]

I was finding the contents of fallback.md hard to understand. I've made an attempt to revise it.

• Change translation key from message to hello because messages is a special keyword in vue-i18n, and then it looks like message might also be special.
• Add summaries atop each section so it's clear what config feature we're discussing.
• Change translation key to Hello, world! to make it clear that you can include special characters in the key -- that's sort of the whole point of this section.
• Revise for English idiom (e.g. "enrich this feature" is not idiomatic).

[kazupon]

Thanks! ❤️