DOC: clarify hsu soiling ratio minimum value in docstring

[Omesh37]

• Closes Possible improvement to pvlib.soiling.hsu documentation #2534
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Summary

Closes #2534

Improved the pvlib.soiling.hsu docstring to clarify that the HSU model
has a minimum soiling ratio of approximately 0.6563, due to the mathematical
limit of the error function in the model equation:
SR = 1 - 0.3437 * erf(0.17 * ω^0.8473)

As erf approaches 1.0 at large inputs, SR cannot drop below 1 - 0.3437 = 0.6563.
Detailed explanation has been moved to a Notes section as suggested in review.

Changes

• Updated Returns docstring in pvlib/soiling.py
• Updated function summary paragraph
• Added whatsnew entry under v0.15.2

[cwhanse]

Thanks Omesh37, this is useful additions to the documentation.

[Omesh37]

Thanks for the review @cwhanse
I've gone through the changes and accepted your suggestion for line 22-23.
Also:

• Moved the detailed content to a Notes section
• Rephrased the valid range sentence as suggested
  Please let me know if ant further changes are needed.

[Omesh37]

Thanks for the review @cwhanse
I have changed and accepted the changes.
I new so please consider it as I made such a silly mistake.
Please let me know if ant further changes are needed.

[cwhanse]

@Omesh37 I copied the pull request template into the first comment. Please answer the two open items.

In the future please include the template in pull requests.

[cwhanse]

And welcome to contributors to pvlib :)

[Omesh37]

Sorry about missing the template @cwhanse! I have now filled out all
the checklist items. This is a documentation-only change so Tests added
and API reference update are not applicable. I will make sure to include
the template from the start in future pull requests.