How to link and then flag related documents when a change is made to one of them?

[Conner McFarlane]

I would like to see if there is a way to functionally link two or more documents (for example specs and implementation plans) so that when a change is made to one there will be some sort of visual cue on the other notifying the viewer that one of the other documents had a change. Then this cue can be acknowledged once the changes have been addressed in the corresponding documents.