Doc update (or functionality fix) is required in pyirf.sensitivity.relative_sensitivity

[mexanick]

The docstring of the method says:

    ----------
    n_on: int or array-like
        Number of signal-like events for the on observations
    n_off: int or array-like
        Number of signal-like events for the off observations

However the method itself can't work with arrays, only with scalars. The biggest issue with that is in scipy.optimize.brentq root finding algo, so fixing might be not straightforward. In case of leaving as is a doc update is needed.