doc: don't suggest setEncoding for binary streams

Removed an incorrect reference to the use of setEncoding(null) as the
proper way to handling binary streams or to disable encoding, and
explained that the default encoding is "no encoding", and that this is
the correct approach for dealing with binary data via Buffers.

PR-URL: https://github.com/nodejs/node/pull/11363
Fixes: https://github.com/nodejs/node/issues/11352
Refs: https://github.com/nodejs/node/issues/11316


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>