Document why Unsafe browser don't access the data of amnesia user

Originally created by @emmapeel on #7772 (Redmine)

It seems people with not enough knowledge doesn’t understand the jail of the Unsafe browser, and find it frustrating not to be able to access ‘Desktop images’.

Maybe https://tails.boum.org/doc/anonymous_internet/unsafe_browser/index.en.html can be updated with an easy explanation.

Related issues

• Related to #7774