Making multiprocessing.pool.Pool.map default to a chunksize of 1

I made a PR for fixing the documentation related to the map method in multiprocessing.pool.Pool. The stackoverflow link that was referenced in the issue contained confusion that imap defaults to a chunksize of 1 whereas map has no default.

Is there a reason why they differ? And if not, shouldn’t map also default to 1 to make things more intuitive?