Document composite transforms with no input and/or output

[damccorm]

Update the programming guide's Composite Transforms section:

• When to use PBegin/PDone in Java (no input/output PCollections)
• Similarly for Python: PBegin/None, and update PBegin pydoc string

Discussion: https://lists.apache.org/thread.html/r5cad76ea7b5b8404d159dd14c24964242b1d4b49787d6aa6c162d948%40%3Cdev.beam.apache.org%3E

Imported from Jira BEAM-10495. Original Jira may contain additional context.
Reported by: udim.