Document how to avoid mypy reporting

When using `.hvplot` on a dataframe or series, `mypy` will report a `operator` missing problem. See https://mypy.readthedocs.io/en/stable/error_code_list.html#code-operator.

You can solve this by adding `# type: ignore[operator]`. This is quite cumbersome to add many places in your docs.

It would be very nice if this was discussed and best practices documented in the documentation.