doc: improve `stream.Writable` ctor encoding option docs

- Document `defaultEncoding`. This was previously undocumented.
- Clarify `decodeStrings`. In particular, the previous description was
  talking about decoding, which matches the unfortunate option name,
  but what actually happens is usually refererred to as encoding
  a string into a binary form.

PR-URL: https://github.com/nodejs/node/pull/23246


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>