DOC: remove incorrect example from __add__ et al docstring
Notices while reviewing the doc PRs. We add a similar example to the `__add__` and similar docstrings (based on ``_arith_doc_FRAME`` template) than to `add` (which use the `_flex_doc_FRAME` template), so the example is incorrect, and also not needed for that docstring I think (nobody should directly use `__add__`?) 
So remove it to have less code to maintain.


