doc: clarify N-API version 1

Refs: https://github.com/nodejs/node-addon-api/issues/760



Clarify which version of 8.x in which N-API version 1
matches the shape in later versions like 10.x

Signed-off-by: Michael Dawson <michael_dawson@ca.ibm.com>

PR-URL: https://github.com/nodejs/node/pull/34344


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>