doc: server.listen truncates socket path on unix

Internally it ends up calling `uv_pipe_bind` with the given path which
itself is documented to truncate the path. See
http://docs.libuv.org/en/v1.x/pipe.html#c.uv_pipe_bind

This is NOT a bug, but a restriction of the unix
socket api, as it stores the path in `sockaddr_un.sun_path` (104 chars
on OS X, 108 chars on Linux), see `man unix`.

PR-URL: https://github.com/nodejs/node/pull/6659


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Alexander Makarenko <estliberitas@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>