Clarify `select.kqueue` and `select.kevent` also available on macOS

[aisk]

The document says kqueue() and kevent() are only supported on BSD, but macOS also fully supports them:

cpython/Doc/library/select.rst

Line 103 in 7e3a5a7

(Only supported on BSD.) Returns a kernel queue object; see section

cpython/Doc/library/select.rst

Line 114 in 7e3a5a7

(Only supported on BSD.) Returns a kernel event object; see section

While macOS uses a userland from BSD (FreeBSD in these days I guess?), our documentation consistently treats macOS as a distinct platform from BSD, for example:

cpython/Doc/library/socket.rst

Line 651 in 7e3a5a7

.. availability:: BSD, macOS.

Linked PRs

• gh-143293: Clarify that select.{kqueue,kevent} can be used on macOS #143294