Explain better "type in a bridge"

During the usability tests in August:

• P1, P3, and P4 wondered what to type in the textarea.
• P4 thought that they couldn't send an email because they are not connected yet to the Internet.

We should explain better how to get and enter bridges:

• Explain better the input field.
• [ ] Only display “obfs4 bridges start with the word” on input error. → #18478
• Write "enter" instead of "type", for accessibility reasons.
• Type instead of type in.