doc: describe why new Buffer() is problematic

Existing docs weren't clear on the actual problem. In addition, the text
described 8.0.0 as being a future Node.js release, so adjust language
to reflect that 8.0.0 is in the past (while not losing important
information about what the pre-8.x behaviour was).

PR-URL: https://github.com/nodejs/node/pull/28825


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>