Possible improvement to pvlib.soiling.hsu documentation

[ramaroesilva]

Describe the bug
hsu model documentation mentions that the output

soiling_ratio (Series) – Values between 0 and 1. Equal to 1 - transmission loss.

While the soiling ratio ranges 0-1, broadly speaking, it is also true that this model internally caps its minimum value at 0.6563 (i.e., 1-0.3437). For example, in the kimber model this cap is a user-defined parameter.

To Reproduce
Steps to reproduce the behavior:

1. Go to hsu model documentation
2. Check the description of the returned output

Expected behavior
The documentation could mention that the model implicitly defines a minimum value for the soiling ratio of 0.6563.

Versions:

• pvlib.__version__: 0.13.0

[cwhanse]

+1 to adding the HSU equation and a note to the function docstring.

Keep in mind that hsu and kimber return different quantities. hsu returns the soiling ratio, a fraction of irradiance. kimber returns a daily electrical energy loss. So the cap max_soiling in kimber is different that the unnamed maximum in hsu.