Update Optional Chaining documentation to reflect the return from undefined to true

Per https://github.com/babel/babel/issues/10805 and the subsequent [PR](https://github.com/babel/babel/pull/10806) , When using optional delete to delete a property of a nullish base, the delete operation returns now returns true. Should a distinction be made in the docs to support this change?

If so, I can make the corresponding change to the documentation.