Instruct to start without persistence to be able to delete it or to change the password

In the documentation about how to delete the persistent volume (https://tails.boum.org/doc/first_steps/persistence/delete/index.en.html), we should had a paragraph which say you have to launch a session with the persistence disabled to remove it. This also applies for https://tails.boum.org/doc/first_steps/persistence/change_passphrase/index.en.html

Feature Branch: web/11822-instruct-to-start-persistence-without-password-for-deletion

Original created by @xin on 11822 (Redmine)

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