You can try the eclipseUML Omondo usecase diagram. You can for example write a full documentation directly in the usecase UML model. For example add flows, description, stereotypes and even extension point, rank, priority etc.....
This information belongs to the model element so if you drag and drop the same element into another diagram then you still have this requirement information. Really cool, very easy to use and so powerful !!
The main advantage is that you only have one model shared by the team members and not multiple documents attached to the project. Developers never read documents so it is better to give them information directly inside the IDE they use and Eclipse is the solution for me.