Currently there is not way to express such a logic in the API Blueprint. You can always describe it "by mouth" in the respective responses or under the action itself. For example:
# Resource [/1]
## Action [GET]
Explanation of this action here.
+ Request
...
+ Response 200
Explanation of this response.
+ Headers
...
+ Body
...
+ Response 304
Explanation of this response.
+ Headers
...
Note it is quite likely there will be some limited branching support in the near future in order to support content negotiation.