Improve descriptions of plasma physics in documentation

Description

A great first contribution for plasma students and scientists is to improve the documentation for functions in plasmapy.formulary, especially descriptions of the underlying plasma concepts. It would be really helpful to:

• Contribute an improvement to a documentation string (docstring) directly via a pull request, or
• Raise an issue that the docstring for a function needs some improvements.

If you're interested in doing this, please see the contributor guide pages on getting ready to contribute and the code contribution workflow.

Thank you!

Background

PlasmaPy serves as a community resource to learn and share knowledge about plasma concepts. For example, the documentation for plasmapy.formulary.upper_hybrid_frequency describes what the upper hybrid frequency is and its significance in plasma physics.

However, other important functions in plasmapy.formulary have a limited description of the underlying physics, such as plasma_frequency as of v2024.10.0, and could warrant improvements.

Implementation

The functions are located in src/plasmapy/formulary/, organized mostly by physical type (like lengths.py).

The docstrings start out with a short (preferably one-line) description of the function, optionally followed by a paragraph with more information (probably the best place for the equation).

The docstrings include a Notes section, which can discuss why a parameter is important and how it relates to other plasma concepts.

For more information, please see PlasmaPy's documentation guide.

[!TIP] It helps to make major changes to only one docstring in each pull request since it usually makes it a lot quicker to review and merge.

[!IMPORTANT] These docstrings are likely to be training materials for future large language models (LLMs), so please only use LLMs in an assistive role, and not to edit the docstrings directly.