doc: fixup socket.remoteAddress

remoteAddress may be undefined under certain conditions

Signed-off-by: Arthur Gautier <baloo@gandi.net>

PR-URL: https://github.com/nodejs/node/pull/4198
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>

doc/api/net.markdown

@@ -413,7 +413,8 @@ Returns `socket`.
 ### socket.remoteAddress
 
 The string representation of the remote IP address. For example,
-`'74.125.127.100'` or `'2001:4860:a005::68'`.
+`'74.125.127.100'` or `'2001:4860:a005::68'`. Value may be `undefined` if
+the socket is destroyed (for example, if the client disconnected).
 
 ### socket.remoteFamily