Make it clear that the Yunohost migration tool will update both the Debian system and the Yunohost version

[zeroheure]

Problem

It was not clear for me that the Yunohost tools can migrate both the Debian system and the Yunohost packages. Actualy, while testing the migration, I wrongly understood an error message as "migrate the system first".

Solution

Now the documentation insist on this, in french and english. Text added in the 3 relevant chapters.

PR checklist

• [X] PR finished and ready to be reviewed