[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)
diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index 548e4a6..663e232 100644
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -38,7 +38,7 @@
 
 
 .. function:: run(args, *, stdin=None, input=None, stdout=None, stderr=None,\
-                  shell=False, timeout=None, check=False, \
+                  shell=False, cwd=None, timeout=None, check=False, \
                   encoding=None, errors=None)
 
    Run the command described by *args*.  Wait for command to complete, then
@@ -856,7 +856,7 @@
 subprocess. You can now use :func:`run` in many cases, but lots of existing code
 calls these functions.
 
-.. function:: call(args, *, stdin=None, stdout=None, stderr=None, shell=False, timeout=None)
+.. function:: call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None)
 
    Run the command described by *args*.  Wait for command to complete, then
    return the :attr:`~Popen.returncode` attribute.
@@ -882,7 +882,7 @@
    .. versionchanged:: 3.3
       *timeout* was added.
 
-.. function:: check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False, timeout=None)
+.. function:: check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None)
 
    Run command with arguments.  Wait for command to complete. If the return
    code was zero then return, otherwise raise :exc:`CalledProcessError`. The
@@ -912,7 +912,7 @@
 
 
 .. function:: check_output(args, *, stdin=None, stderr=None, shell=False, \
-                           encoding=None, errors=None, \
+                           cwd=None, encoding=None, errors=None, \
                            universal_newlines=False, timeout=None)
 
    Run command with arguments and return its output.