Add reference to tracking.singleaxis

[RDaxini]

I noticed Discussion #2137 and PR #2146 on wid/omega_ideal
I don't think the variable omega_ideal is mentioned in the cited publication (Anderson and Mikofski, 2020) either so I think it might help to add the original reference of this function for additional context, not just for this variable but also more generally too.

• Closes What does "wid" stand for? #2137, relates to changed w to omega in tracking.py's singleaxis function #2146
• I am familiar with the contributing guidelines
• 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.

Changes summary:
Added reference
Other minor docstring edits e.g. adding `` around function arguments

[kandersolar]

I wonder if switching all notation etc to match one reference would be a better approach than progressively clarifying the current mix of notation. But I think it's clear that this PR is an improvement over the status quo, so fine with me to merge as is.

[kandersolar]

Thanks @RDaxini!