Documentation tab has a text field that includes more detailed descriptions and further information on the data model.
If you have editing rights, you can also see the Edit-button. Click the Edit button to edit 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 / 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.
You can find general information about Markdown notation from, for example, the GitHub instructions: https://guides.github.com/features/mastering-markdown/#syntax