In login-throttled, specify how long you have to wait before logging in

Throttle time will be enough to give an idea of the time to wait.
The actual time seems to be the time since the first of the last n
attempts in time t as per $wgPasswordAttemptThrottle, but it does not
seem too bad if the user gets a new full pool of allowed attempts.

Bug: 46965
Change-Id: Id385be840f340476fbe7a818ff1d05154fe86d68