bpo-31965: fix doc for multiprocessing.connection.Client and Listener (#4304)

* fix doc for multiprocessing.connection.Client

The authenticate argument does not exist on either Client or Listener:
- https://github.com/python/cpython/blob/master/Lib/multiprocessing/connection.py#L483 (master)
- https://github.com/python/cpython/blob/3.6/Lib/multiprocessing/connection.py#L478 (3.6)
- https://github.com/python/cpython/blob/3.5/Lib/multiprocessing/connection.py#L478 (3.5)
- https://github.com/python/cpython/blob/3.4/Lib/multiprocessing/connection.py#L487 (3.4)
The documentation also claimed that these functions will call `current_process().auth_key`, for which I could find no evidence in the code. I rewrote the documentation to reflect the actual behavior.

Also made some small changes to vary sentence structure.