In a Python Toolbox you can create help for the side pane by editing the Item Description for the tool. To do this you can right-click on the tool in your toolbox
Click Item Description which will open a new dialog. Click on Edit and you can edit the details.
These details you add to the Summary will be shown in the help side pane.
The other details will be shown in a browser when you click on Tool Help, or when you open Item Description
Browser shown after clicking "Tool Help" from the tool:
The help for the toolbox and for each tool is stored in
.xml files. There is a separate
.xml file for each tool.
For more info see Documenting a tool in a Python toolbox