Update documentation for address formats to describe AF_UNIX
surrogateescape handling and Linux abstract namespace.

diff --git a/Doc/library/socket.rst b/Doc/library/socket.rst
--- a/Doc/library/socket.rst
+++ b/Doc/library/socket.rst
@@ -34,8 +34,15 @@ in the C interface: as with :meth:`read`
 files, buffer allocation on receive operations is automatic, and buffer length
 is implicit on send operations.
 
-Socket addresses are represented as follows: A single string is used for the
-:const:`AF_UNIX` address family. A pair ``(host, port)`` is used for the
+The address format required by a particular socket object is
+automatically selected based on the address family specified when the
+socket object was created.  Socket addresses are represented as follows:
+
+``None`` does not represent an address in any family, but is returned
+when the operating system does not return an address structure, as is
+often the case when receiving data on a connected socket.
+
+A pair ``(host, port)`` is used for the
 :const:`AF_INET` address family, where *host* is a string representing either a
 hostname in Internet domain notation like ``'daring.cwi.nl'`` or an IPv4 address
 like ``'100.50.200.5'``, and *port* is an integral port number. For
@@ -44,10 +51,7 @@ scopeid)`` is used, where *flowinfo* and
 and ``sin6_scope_id`` member in :const:`struct sockaddr_in6` in C. For
 :mod:`socket` module methods, *flowinfo* and *scopeid* can be omitted just for
 backward compatibility. Note, however, omission of *scopeid* can cause problems
-in manipulating scoped IPv6 addresses. Other address families are currently not
-supported. The address format required by a particular socket object is
-automatically selected based on the address family specified when the socket
-object was created.
+in manipulating scoped IPv6 addresses.
 
 For IPv4 addresses, two special forms are accepted instead of a host address:
 the empty string represents :const:`INADDR_ANY`, and the string
@@ -62,6 +66,24 @@ differently into an actual IPv4/v6 addre
 resolution and/or the host configuration.  For deterministic behavior use a
 numeric address in *host* portion.
 
+The address of an :const:`AF_UNIX` socket bound to a file system node
+is represented as a string, using the file system encoding and the
+``'surrogateescape'`` error handler (see :pep:`383`).  An address in
+Linux's abstract namespace is returned as a :class:`bytes` object with
+an initial null byte; note that sockets in this namespace can
+communicate with normal file system sockets, so programs intended to
+run on Linux may need to handle both types of address.  A string,
+:class:`bytes` or buffer-compatible object can be used for either type
+of address when passing it as an argument.  The address of an unbound
+:const:`AF_UNIX` socket is usually returned as an empty string or
+``None``; some platforms can return the latter even in contexts where
+a real address would normally be expected.
+
+.. versionchanged:: XXX
+   Previously, :const:`AF_UNIX` socket paths were assumed to use UTF-8
+   encoding, and objects supporting the read-write buffer interface
+   were not accepted.
+
 AF_NETLINK sockets are represented as  pairs ``pid, groups``.