PPS: list_pipeline has no `pipeline_name` requirement in 6.x (parity with core Pachyderm 1.x)

[echohack]

Hello,

This morning I was answering some questions for a customer that is using Pachyderm 1.x and discovered that the docstring for list_pipeline is incorrect. It says it requires the pipeline_name but it clearly does not in the method signature itself.

https://github.com/pachyderm/python-pachyderm/blob/v6.x/src/python_pachyderm/mixin/pps.py#L550

This appears to be a docs issue for 6.x python-pachyderm and has caused some customer confusion.