bpo-41857: Document timeout arguments in select module

The docstring for the poll() methods of the select.poll[1] and
select.devpoll[2] classes did not describe the expected units of the
timeout argument (milliseconds). Previously this information was only
available from the online documentation.

This is a potential source of confusion, because other poll() methods in
the select module, such as select.epoll and select.kqueue objects, use
units of seconds (and were documented).

Include the units in the docstring description so that this information
is at least available from interactive help.

[1] https://docs.python.org/3/library/select.html#select.poll.poll
[2] https://docs.python.org/3/library/select.html#select.devpoll.poll

Author: zaneb

Modules/clinic/selectmodule.c.h

@@ -564,6 +564,8 @@ select_poll_unregister_impl(pollObject *self, int fd)
 select.poll.poll
 
     timeout as timeout_obj: object = None
+      The maximum time to wait in milliseconds, or else None (or a negative
+      value) to wait indefinitely.
     /
 
 Polls the set of registered file descriptors.
@@ -574,7 +576,7 @@ report, as a list of (fd, event) 2-tuples.
 
 static PyObject *
 select_poll_poll_impl(pollObject *self, PyObject *timeout_obj)
-/*[clinic end generated code: output=876e837d193ed7e4 input=7a446ed45189e894]*/
+/*[clinic end generated code: output=876e837d193ed7e4 input=c2f6953ec45e5622]*/
 {
     PyObject *result_list = NULL;
     int poll_result, i, j;
@@ -895,6 +897,8 @@ select_devpoll_unregister_impl(devpollObject *self, int fd)
 /*[clinic input]
 select.devpoll.poll
     timeout as timeout_obj: object = None
+      The maximum time to wait in milliseconds, or else None (or a negative
+      value) to wait indefinitely.
     /
 
 Polls the set of registered file descriptors.
@@ -905,7 +909,7 @@ report, as a list of (fd, event) 2-tuples.
 
 static PyObject *
 select_devpoll_poll_impl(devpollObject *self, PyObject *timeout_obj)
-/*[clinic end generated code: output=2654e5457cca0b3c input=fd0db698d84f0333]*/
+/*[clinic end generated code: output=2654e5457cca0b3c input=3c3f0a355ec2bedb]*/
 {
     struct dvpoll dvp;
     PyObject *result_list = NULL;