Clarify the relationship between USB images and upgrade

Originally created by @Anonymous on #16325 (Redmine)

We received one tester report that proposes to improve the documentation:

“Please tell (on the ”$“:https://tails.boum.org/home/index.en.html page)
whether the USB-image is intended be used for the first installation
only and not for updates of the TAILS?”

This sentence is not super clear, I understand the person wants to know if this is an intermediary stick or not, and maybe we need to make this clear to users who are used to have two sticks to install. I let you judge on what you think is appropriate.

Parent Task: #15292 (closed)