doc: improve Release Types text in README

Simplify text and improve clarity.

PR-URL: https://github.com/nodejs/node/pull/23190


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>