doc: clarify role of writable.cork()

The syntax of the sentence describing the role of writable.cork() was
unclear. This rephrase aims to make the distinction between writing
to the buffer and draining immediately to the underlying destination
clearer - while keeping performance considerations clearly in mind.

PR-URL: https://github.com/nodejs/node/pull/30442


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>