Python Trigger Functions are missing PyDoc

[kennknowles]

See for instance the last classes in this page:

https://beam.apache.org/documentation/sdks/pydoc/2.4.0/apache_beam.transforms.trigger.html#apache_beam.transforms.trigger.AfterCount

Imported from Jira BEAM-4482. Original Jira may contain additional context.
Reported by: pabloem.