update spectral_factor_sapm docs

[RDaxini]

Is your feature request related to a problem? Please describe.
I think the documentation for the spectral_factor_sapm function is lacking detail. There is a comment directing users to sapm() for further detail, but actually I'm not 100% sure what that is--- is it pvsystem.sapm?

Describe the solution you'd like
I'd like to update the docs to a similar level of detail as the other spectral_factor docs. At a minimum:

• hyperlink the sapm module referenced currently (this page?)
• Add more information on the model development and other context, including the equation
• Add references, e.g. 1, e.g. 2, e.g. 3

Describe alternatives you've considered
I understand that the SAPM air mass function is not commonly used nowadays, but as it is still in pvlib I think it would still add value to substantiate the docs where the function is found. I'd like to create a PR for these changes but I think I should only go ahead if I get a couple of +1s for this agreeing it'd add value.

Any other suggestions and feedback always more than welcome.