doc: fix Writable.write callback description

Clarifies a userland invariant until a better
solution can be found.

Also moves a misplaced sentence from _write to
write.

Refs: https://github.com/nodejs/node/pull/31756
Refs: https://github.com/nodejs/node/pull/31765

PR-URL: https://github.com/nodejs/node/pull/31812


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