I don't think so.
The options you have to documentate code:
- Entity level Documentation as you said
- Description field at Service Level
- Each Input parameter has it's own Description section
- Output has it's own Description section
- Have a Description field
- Have a Description Section
Just one last perk, when you save an entity on the composer if you click on the button next to Save you can write a comment on what's modified.
As you can see TW it's well designed to documentate everything, where it fails it's then if you want to generate a "printable" version of everything you had write on previous parts. But you can enter on "Server" view and see everything you documented: "yourServer"/Thingworx/Server