stream: complete migration to internal/errors

Complete the migration to the new error system of _stream_readable
and _stream_writable. Adds the corresponding documentation.

PR-URL: https://github.com/nodejs/node/pull/16589


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>