I don't think there's a clear overview of the changes in 2.2. There's a UML 2.2 specification document with change bars: it marks the changed sections as opposed to version 2.1.2. It's really the best way to view those changes - again, there's no real overview, at least none that I could find. You can view the document (PDF) here.
I've skimmed the document a bit. They're mostly minor changes. Some of the changes I came across you can quickly find in:
- 8.3 (Class Descriptions)
- 8.3.2 (ComponentRealization)
- 10.2 (Deployments - Abstract Syntax)
- 11.2 (Abstract Syntax)
- 11.3.12 (ClearStructuralFeatureAction)
- 11.3.19 (InputPin)
- 11.3.33 (ReadLinkAction)
- 11.3.47 (StartObjectBehaviorAction)
- 11.3.55 (WriteStructuralFeatureAction)
- 15.2 (State Machines - Abstract syntax)