doc: clarify allowed encoding parameter types

This fixes the incorrect enumerations of their possible values, which
weren't up to date with the values actually supported. Also renamed
two arguments that used "format" when they meant "encoding".

PR-URL: https://github.com/nodejs/node/pull/24230


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>