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

[echedey-ls]

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 #2253 (comment)