Missing explanation of mounting type in document

[miettal]

Describe the bug

pvlib.temperature.sapm_cell and pvlib.temperature.sapm_module can be specified coefficient a and b.
and It's depends on module and mounting.
In document, mounting method is listed, but is not described detail.

On the other hand, mounting method detail was described in NREL SAM Help System, and also mentioned in NREL SAM forum.
help system code: https://github.com/NREL/SAM/blob/master/deploy/runtime/help/html/pv_module.htm
forum: https://sam.nrel.gov/forum/forum-general/1040-mount-types.html?start=0

This is missing of mounting type explanation in the document.

To Reproduce

Expected behavior

Explanations of mounting types are described in the document.

Screenshots

Versions:

0.9.0

Additional context

(if this issue have to be a Support question, I re-ask in StackOverflow)

[cwhanse]

@miettal thanks for pointing out this shortcoming in the documentation. I think we can add text to each function's docstring. The racking_model keyword for pvlib.pvsystem.PVSystem doesn't provide much explanation either, but that could point to the documentation for functions in pvlib.temperature. Pull request welcome.