doc: clarify that fs streams expect blocking fd

This clarifies that fs.createReadStream and fs.createWriteStream, when
passed a fd, expects the fd to be blocking, and suggests net.Socket as
an alternative.

PR-URL: https://github.com/nodejs/node/pull/3641


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>