Jesse Seymour discusses a few forms of documentation for a data warehouse:
Extended properties are a great way to internally document the data warehouse. The key advantage here is that the values of these extended properties can be retrieved with a T-SQL query. This allows us to access this information with a view as needed. My favorite method of using this is to create an SSRS report that end users can run to look up the attributes and comments I store in the extended property. Data warehouse tools take some of the pain out of the process. Unfortunately, not all tools support use of extended properties. Make sure your tool does or consider changing tools. Be sure to document the names and use cases for each property you create. Consistency is the key to the value here.
I’ve never been a big fan of extended properties, mostly because I typically don’t work with tools which expose that information easily. Regardless, there are other important forms of documentation, so read on.