> by classprop do you mean classprop is a multilink to a Class?

No. It is just a link.

It may be undocumented, but in this case 
context.documents.sorted('classprop')
sorts the documents list by the order property of the (linked) classprop property of the elements in this list. [*]

And in my opinion, this is the only sensible definition of the above command (in real life situations 'classprop' can be 'importance', for example: and instead of hard-coded numbers we use a Class() for representing the possible values of importance). Otherwise, this operation should be undefined. :)

And the same "bug" occurs in [*] as in issue2551122: The order property is always treated as string.