doc: update context-aware section of addon doc

Replace the portion of the context-aware addon example that advocates
for managing the life cycle of the per-addon-instance data using a
weak reference with code that illustrates how to manage its life cycle
using an environment cleanup hook.

Signed-off-by: Gabriel Schulhof <gabriel.schulhof@intel.com>
PR-URL: https://github.com/nodejs/node/pull/28659


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>