Update copy for the "Add a bridge manually" modal

Some users remove information from the bridge input area because the placeholder text says: "type address:port (one per line)".

Normal bridges would work, but it is safer to add the fingerprint as well.

Obfs4 bridges do not work. We have had cases of users contacting support because they remove fingerprint, "cert" and "iat_mode" from obfs4 bridge lines, and then they cannot connect.