docs: improve Writable/Transform stream docs

Slightly fix and supplement the documentation for Writable#write and
Transform#transform methods. Make it more consistent. Mention passing
'buffer' as an encoding param in case chunk is a Buffer.

PR: https://github.com/iojs/io.js/pull/142


Reviewed-by: Bert Belder <bertbelder@gmail.com>
Reviewed-by: Rod Vagg <rod@vagg.org>