doc: update notes about GCM decryption

Given that https://github.com/nodejs/node/pull/17825 and
https://github.com/nodejs/node/pull/20039 have landed on master, this
statement is no longer true.

PR-URL: https://github.com/nodejs/node/pull/21445


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>