Here is a message which points out the two possible approaches:
The first approach is to sort the collection by putting it into an array. The second is to use a NotesviewentryCollection instead of a document collection.
In the array sorting approach, the poster makes reference to some code in the sandbox which I have not seen. If that doesn't work, I know that Teamstudio (www.teamstudio.com) markets a code library from Mental Notes called "Script Coach" that contains, among many other useful code snippets, a function that sorts a document collection using this approach, and I have used it myself with great results.