Explain better how to type a bridge (#18597)

Closes #18597 (closed)

During the usability tests in August, people were very confused about the procedure to follow here.

The phrase "obfs4 bridges start with the word 'obfs4'" made some participants believe that they had to start typing 'obfs4' and wondered about what else to type.

I'm clarifying the procedure with the phone. I used "type" on purpose in the phone example to be more explicit to the vast majority of people who will indeed type it on their keyboard.

People were concerned about the security aspects of asking bridges over email. Giving it as an example in the UI could help people feel more reassured. I'm not sure that we should be more verbose in the UI itself and the doc already explains it better.

Nowadays bridges@torproject.org and (bridges.torproject.org) only return obfs4 by default, so we don't need to warn about what type of bridge to use. We'll get back to it if bridges@torproject.org or bridges.torproject.org ever returns by default a type of bridge that is not supported in Tails.