Documentation tab has a text field that may have more comprehensive descriptions and further information of the data model.
If you have editing rights, you can see also the Edit-button. Click the Edit button to start editing the text.
In practice, the field has an add-in, Markdown-application, which allows you to edit the layout of the text, for example add titles, create normal and bold text, add links to pictures etc. While creating or editing the text, you can see the preview of an actual published text under Preview-title.
After you have added the information, click the Save-button. The field will show the information you just added.
General information about Markdown-notation you can find for example from the instructions of GitHub: https://guides.github.com/features/mastering-markdown/#syntax.