Correct misleading docstring on confusing hub port vs hub port range (#2224)
Correct misleading docstring on confusing hub port vs hub port range (#2224)

Although the naming and previous docstring suggest that the two parameters
work together to define a single port which either is specified explicitly
or comes from the supplied range, this is not true.

Two listening ports are used:

* one for the UDP receiver for message from workers

and

* one for ZMQ queue from from the htex interchange (or any other executor
  that wants it)

The UDP port is either `hub_port` or an arbitrary system-allocated port,
ignoring hub_port_range.

The ZMQ queue port is always chosen from hub_port_range, which has a
default value so a user is not required to specify it.
Parsl/parsl,Parsl/parsl,Parsl/parsl,Parsl/parsl