doc: describe why new Buffer() is problematic
Existing docs weren't clear on the actual problem. In addition, the text described v8.0.0 as being a future Node.js release, so adjust language to reflect that v8.0.0 is in the past (while not losing important information about what the pre-8.x behaviour was).
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines