doc: clarify create{Read,Write}Stream fd option

Clarify the fd option: it is preferred to the path parameter, omits
the "open" event if given, and is available on WriteStreams as well.

PR-URL: https://github.com/joyent/node/issues/7707
Fixes: https://github.com/joyent/node/issues/7707
Fixes: https://github.com/joyent/node/issues/7708
Fixes: https://github.com/joyent/node/issues/4367
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>