Document time parsing and warn if time in milliseconds is out of range

[melissalinkert]

Fixes #4142.

This is mainly meant to clarify the expectations of decodeTime(int), namely that it takes the 32-bit int stored in a Metamorph file for the time of day in milliseconds, and converts to a human-readable string timestamp. The actual date is stored separately, so the number of milliseconds should represent less than 24 hours.

The only change in behavior should be that there is a warning if the number of milliseconds is outside the bounds of the number of milliseconds in a day (which is much narrower than int32 range). That shouldn't happen in practice with Metamorph data, but is included because this is a public method. I could see a case here for throwing IllegalArgumentException instead.