Instruct about the possibility of creating a persistent storage when there is none

From https://gitlab.tails.boum.org/tails/blueprints/-/wikis/Persistent-Storage-redesign:

Such a message would help solving 2 issues from the user testing of the Additional Software beta:

• P3 expected the persistent storage to appear in Tails Greeter (before she started the session and created one).
• 3 participants out of 5 missed the new field “Encrypted Persistent Storage” the first time it appeared. See also #15573 (closed).

I'm not sure the image will look good on the actual screen.

During #18784 (closed), P3 and P4 wouldn't have been able to figure out by themselves that they can create a Persistent Storage to store the Wi-Fi password.