doc: update writable.write return value

Fix the explanation which stated that write() would return false if
highWaterMark is exceeded to correctly state that false is returned
once highWaterMark is reached. See #9247.

PR-URL: https://github.com/nodejs/node/pull/10582


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>