Recently, I have been spending time looking at the Swagger 2.0 Specification. There XML representation and Enumerations are tricky. Here is an example of a Swagger Enumeration in YAML.
You can test the YAML https://gist.githubusercontent.com/prb112/59b9cb4ad3d2df19911c/raw/7db2a16b353d8dde5bb3743001f46f7b67b6c431/enumeration.yaml at http://petstore.swagger.io/#!/default/get_demo

Can I add description to each enum value ?
yes, you can. I think it’s value
Can we have re-usable enums ? i.e Enums defined at a placed and reuse them ?
I personally have used them in the model. You should be able to reference them in a parameter