Detailed explanation of `racking_module` and `module_type` in `PVSystem` docstring

[matsuobasho]

Is your feature request related to a problem? Please describe.
Without prior knowledge, the racking_model and the module_type parameters are not properly explained, especially with respect to how their combination is used to select a temperature model.

Describe the solution you'd like
A detailed description racking_model and module_type and how/why certain combinations are then used to select a SAPM or PVSyst temperature model. This description should be in the notes section of PVSystem and Array.

It would also be useful to include a brief version of this description in the PVSystem user guide.

Also, note that currently the docstring of the racking_model argument in both PVSystem and FixedMount is incomplete - it is missing the freestanding option. This needs to be fixed.

Describe alternatives you've considered
I've searched the codebase and tutorials for an explanation.

[matsuobasho]

In addition to the racking_model incompleteness mentioned above, the docstring states that default is 'open_rack', but the code has the default as None. Which is one the intended default?

[cwhanse]

IMO None is the intended default and the docstring should be corrected.