Add note during secret import about BIP-39 passphrases

[AndreasGassmann]

In AirGap, the BIP-39 passphrase can be set when adding an account, not when importing a secret.

In other wallets, the BIP-39 passphrase often have to be set during the mnemonic import step. We should add a note to the UI to let the users know that we do support BIP39 passphrases, but it has to be provided in a different step in the process.

[TangMonk]

Indeed