doc: `readable.push(undefined)` in non-object mode

`readable.push()` supports `undefined` in non-object mode, but it was
not previously documented.

PR-URL: https://github.com/nodejs/node/pull/18283


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