The assembly does not contain the actual comment documentation. You need to generate an XML file that contains all your documentation.
Go into your projects Settings, on the Build tab Check "XML Documentation File":
Now when you compile your project, a nice XML file will be produced containing the documentation.
In your other project, you need to reference the DLL and make sure that the XML has the same name as your assembly with an XML extension instead of DLL, and is in the same directory as your referenced DLL. For example:
Assembly: MyProject.dll
XML: MyProject.xml
You can learn more about XML documentation at MSDN.