[3.6] bpo-30420: List cwd parameter in subprocess convenience APIs (GH-1685) (GH...

[3.6] bpo-30420: List cwd parameter in subprocess convenience APIs (GH-1685) (GH-2253)

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.
(cherry picked from commit 368cf1d20630498ca7939069a05d744fabb570aa)