doc: reword Experimental stability index

Reword the explanation of the Experimental stability index for brevity
and clarity.

PR-URL: https://github.com/nodejs/node/pull/27879


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>