Improve prompt when replying to an email

[micz]

When using the 'reply to this' option by sending the entire text of the email, it's needed to study a prompt to indicate how emails are structured, or at least that there is quoted text. Different prompts need to be tested to verify their effectiveness.

Originally reported here: https://addons.thunderbird.net/it/thunderbird/addon/thunderai/reviews/1173882/