stream: add comments to pipeline implementation

Fixes: https://github.com/nodejs/node/issues/32039

PR-URL: https://github.com/nodejs/node/pull/32042


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>