Document "racking model" for temperature models

[cwhanse]

• Closes Missing explanation of mounting type in document #1323
• I am familiar with the contributing guidelines
• [ ] Tests added
• [ ] Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Also removes the no-longer correct default from the docstring of PVSystem

[RDaxini]

I think we have some overlap here with #2232, e.g. with pvlib/pvsystem.py

[cwhanse]

I think we have some overlap here with #2232, e.g. with pvlib/pvsystem.py

Right. I'll sort that. We should merge #2232 first then fix this PR.