Defining parameter descriptions for Python Toolbox help?

I can see that the help text is defined with the classes self.description attribute.

This is where you are going wrong. In the help page Documenting a tool in a Python toolbox it says:

For Python toolboxes, the documentation for the toolbox and tools are stored in .xml files that are associated with the toolbox and tools by name. The help for each tool will be stored in a separate .xml file.

This means that you cannot set help text from within the .pyt file itself. This makes sense when you consider that the help text is not plain ASCII but rich text that can include formatting, bullets, and images.

Fortunately, Python supports for reading and writing XML, so you should be able to dynamically edit the help text from a separate script.

You can define messages for each parameter by following these steps:

1. Open ArcCatalog and highlight the Script in the catalog tree
2. Select the Description tab
3. Click the Edit button
4. Click the down arrow next to each parameter and type in a message
5. Finally, save edits by clicking the Save button

When a users clicks within a script parameter now they will see your message.

If I understand correctly, you want to add Help texts to your functions. That can be done by right-clicking your tool from the toolbox window and click Item description and then Edit at the top. It'll allow you to add a description for each parameter, which will show up in the help section when clicking the parameter.

If you are in Arcmap 10.0 (at least earlier SP's) you need to do this from the ArcCatalog window (browse to your toolbox) within Arcmap for it to work.