doc: document backlog for server.listen() variants

The backlog parameter is supported by all variations of
net.Server.listen(), but wasn't consistently documented. This
commit brings the documentation into a more consistent state.

Refs: https://github.com/nodejs/node/pull/4056
PR-URL: https://github.com/nodejs/node/pull/4025


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>