Documentation: Misleading default value in gammapy.estimators.SensitivityEstimator of spectrum=None

[mdpunch]

Gammapy version
gammapy v1.1

Bug description
In the documentation of gammapy.estimators.SensitivityEstimator:
https://docs.gammapy.org/1.1/api/gammapy.estimators.SensitivityEstimator.html
... the default for spectrum is given as None

But as can be seen in the source code, this becomes:

        if spectrum is None:
            spectrum = PowerLawSpectralModel(index=2, amplitude="1 cm-2 s-1 TeV-1")

This might mislead, for the unaware, that the sensitivity could be applied to very soft spectrum sources.

Expected behavior

It would be clearer to specify what the default behaviour for spectrum is, e.g. by changing

   Parameters
    ----------
    spectrum : `SpectralModel`
        Spectral model assumption

to

   Parameters
    ----------
    spectrum : `SpectralModel`
        Spectral model assumption, default Power Law with index 2

(note, some CTA papers have used other assumptions, e.g. https://arxiv.org/pdf/1509.01943.pdf "Real-Time Analysis sensitivity evaluation of the Cherenkov Telescope Array" which has "The MC simulation assumes a source with a power law spectrum and a Crab-like index located at the center of the field of view and observed at a zenith angle of 20 degrees.")

[mdpunch]

Made a pull request with this...