Model help can be written for any model by clicking on the Edit model help button () within the Processing modeler window. This will open the Help editor window that has three panels. At the top is an HTML page with placeholders for the Algorithm description, Input parameters, and Outputs sections. At the bottom-left corner, there is an element selection box and there is a box for entering text at the bottom-right corner. To edit an element, select it in the Select element to edit box. Once it is selected, use the Element description box to type a description or necessary documentation. Click on OK when finished:

This help information will then be available on the Help tab when the tool is in execution mode: