bpo-30420: List cwd parameter in subprocess convenience APIs (GH-1685)

Partially clarify the subprocess convenience API documentation by
explicitly listing the `cwd` parameter in their abbreviated signatures.

While this has been merged as an improvement, it doesn't fully
resolve the issue, as the `cwd` should also be covered in the
"Frequently Used Arguments" section, and the fact these APIs
pass unlisted keyword arguments down to the lower level APIs
is currently still unclear.