add: info about account being stored locally and post history not being synced

[plebeius-eth]

Some users have complained about the account post history not being synced across clients, while using the same account file with each client. The account settings should specify that accounts can't be synced yet, and they are stored locally in the website's local storage.

[plebeius-eth]

I think it should say "warning: the account is stored locally ({storageLocation}) and cannot yet be synced across devices" where storageLocation is either the web client hostname, "this desktop app" for the electron client and "this mobile app" for the APK.