It could be possible to do something interesting in this space, where UML can be used generate template code, and later on, another tool could extract UML from the code, compare it to the baseline, and flag any discrepancies. From there, you can either sign off on the discrepancies (and replace your hand-made UML with the extracted one) or fix your code. Bit of a kludge, but at least automatic verification is possible unlike documentation
If only it were that simple but the code is so expressive that you can't really create the UML as easily from it as the other way around. You just can do too much stuff in code that the UML generator would just not understand at all. Or you'd have to basically code in a very specific manner. Not fun. Of course since I last tried it they probably got better at it.
I even remember back in university you'd have to write your custom code _in_ the UML tools dialogs if you didn't want it to be overwritten next time you tried to generate code. Of course these were just simple text boxes. Horrible dev experience.
