ModelChain: Key `"first_solar_spectral_coefficients"` in `module_parameters` is not well-documented.

> Using the key "first_solar_spectral_coefficients" in `module_parameters` is not well-documented. A user would have to look at `PVSystem.first_solar_spectral_loss` method to know to do this.

_Originally posted by @cwhanse in https://github.com/pvlib/pvlib-python/pull/2253#discussion_r1797285310_
            