bpo-39128: Added algorithm description (GH-18624)

GH- [bpo-39128](https://bugs.python.org/issue39128): happy eyeballs description

GH- [3.9] 39128 - happy eyeballs description (GH-18624)
GH- [3.8] 39128 - happy eyeballs description (GH-18624)

https://bugs.python.org/issue39128
(cherry picked from commit 8af4712)

Co-authored-by: idomic <[email protected]>

Author: miss-islington

Doc/library/asyncio-eventloop.rst

@@ -439,6 +439,17 @@ Opening network connections
 
       Added the *happy_eyeballs_delay* and *interleave* parameters.
 
+      Happy Eyeballs Algorithm: Success with Dual-Stack Hosts.
+      When a server's IPv4 path and protocol are working, but the server's
+      IPv6 path and protocol are not working, a dual-stack client
+      application experiences significant connection delay compared to an
+      IPv4-only client.  This is undesirable because it causes the dual-
+      stack client to have a worse user experience.  This document
+      specifies requirements for algorithms that reduce this user-visible
+      delay and provides an algorithm.
+
+      For more information: https://tools.ietf.org/html/rfc6555
+
    .. versionadded:: 3.7
 
       The *ssl_handshake_timeout* parameter.