DOC: use longwave terminology in Fainman radiative model

[kumaradityaapril]

• Closes Use "long-wave" rather than "infra-red" to refer to thermal radiation. #2653
• 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.
• New code is fully documented.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels and Milestone are assigned

This pull request standardizes terminology by using "longwave (thermal)"
instead of "infrared" when referring to thermal radiation in the Faiman
radiative temperature model. This is a documentation-only change and does
not affect behavior or the public API.