Issue #800 - Add formula and reference to WIS documentation

Description

This PR closes #800.

[x] My PR is based on a package issue and I have explicitly linked it.
[x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
[x] I have tested my changes locally.
[x] I have added or updated unit tests where necessary.
[x] I have updated the documentation if required.
[x] I have built the package locally and run rebuilt docs using roxygen2.
[x] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
[ ] I have added a news item linked to this PR.
[ ] I have reviewed CI checks for this PR and addressed them as far as I am able.