doc: de-emphasize wrapping in napi_define_class

Change the documentation for `napi_define_class` in such a way that
it mentions wrapping C++ class instances as a possible use for the API,
rather than making the assumption that it is the use case for the API.

Signed-off-by: Gabriel Schulhof <gabriel.schulhof@intel.com>
Co-authored-by: Rich Trott <rtrott@gmail.com>
Fixes: https://github.com/nodejs/node/issues/36150
PR-URL: https://github.com/nodejs/node/pull/36159


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>