Document support for `EpochsSpectrum/TFR` data in `phase_slope_index`

[tsbinns]

Describe the problem

spectral_connectivity_epochs now supports EpochsSpectrum/TFR objects. phase_slope_index calls spectral_connectivity_epochs for the bulk of its computations, but does not officially support EpochsSpectrum/TFR data.

Describe your solution

Update phase_slope_index docs to state that EpochsSpectrum/TFR data is supported. There would need to be a small change in the logic for how the spectral/spectro-temporal nature of the connectivity is handled (use the connectivity object properties rather than the mode parameter).

Describe possible alternatives

N/A

Additional context

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