doc: handle backpressure when write() return false

The doc specified that writable.write() was advisory only. However,
ignoring that value might lead to memory leaks. This PR specifies that
behavior. Moreover, it adds an example on how to listen for the 'drain'
event correctly.

See: https://github.com/nodejs/node/commit/f347dad0b7b1787092cca88789b77eb3def2d319
PR-URL: https://github.com/nodejs/node/pull/10631


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>