stream: remove TODO and add a description instead

After looking into this it turned out that these two errors are
sanity checks that should not be reached. It is unfortunate that
we assigned error codes for these but changing it into an assertion
seems to be a hassle for `readable-streams`.

PR-URL: https://github.com/nodejs/node/pull/27086


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