doc: note in docs about missing interfaces

Update the documentation for `os.networkInterfaces` to note that any
interface that has not been assigned an address will not be displayed in
the results

Fixes: https://github.com/iojs/io.js/issues/498
PR-URL: https://github.com/iojs/io.js/pull/543


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>