Datamodel's description of `frameobject` missing the new `f_generator` attribute

The `f_generator` attribute was added in https://github.com/python/cpython/pull/124640, and is covered in `inspection` doc page, but it was omitted from the  [datamodel page.](https://docs.python.org/dev/reference/datamodel.html#index-70)


### Linked PRs
* gh-138540
* gh-138553