doc: howto decode buffers extending from Writable

Improved stream documentation with an example of how to decode buffers
to strings within a custom Writable.

Fixes: https://github.com/nodejs/node/issues/15369
PR-URL: https://github.com/nodejs/node/pull/16403


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>