[3.15] gh-110167: Document the increase of test.support.LOOPBACK_TIMEOUT to 10 seconds (GH-155571) (#155583)

Co-authored-by: Karolina Surma <33810531+befeleme@users.noreply.github.com>

Author: miss-islington

Doc/library/test.rst

@@ -285,7 +285,7 @@ The :mod:`!test.support` module defines the following constants:
    :meth:`~socket.socket.recv` and :meth:`~socket.socket.send` methods of
    :class:`socket.socket`.
 
-   Its default value is 5 seconds.
+   Its default value is 10 seconds.
 
    See also :data:`INTERNET_TIMEOUT`.