Better explain email aliases in the Thundermail context

[rtanglao]

• Document that there is 1 Thundermail inbox and all the email alias emails for the 15 Thundermail aliases goes to that 1 inbox. Which means you need to filter in your email client if you want to separate the emails for your aliases.

Checklist of things to write

• clarify in Thunderbird for Desktop and Thundermail
• clarify in Thundermail and Thunderbird for Android