docs: clarify HTTPException docs

[ambergristle]

Motivations

Questions have come in on the Discord server from users who interpreted the docs to mean that HTTPException must be used. There are also some implementation details and "gotchas" that the docs didn't cover.

What's Changed

• clarify that HTTPException is thrown by Hono ecosystem code, but that devs can throw it themselves
• specify why HTTPException might be used instead of other errors
• add context about response types and header behavior
• add call-out about custom response status
• add info about onError and notFound behavior in sub-routes

[yusukebe]

LGTM!

[yusukebe]

@ambergristle

Nice! I think this is perfect. Let's merge. Thanks!