My preference is for the latter. I disagree with John's last comment. Specifically, I think it is easier to have the docs exist once right next to the parameter they refer to, rather than have the docs all clustered at the top with the names repeated (or out-of-sync) and no immediate indication of the type.
Kind of like having a docblock above each method, rather than just having them all at the top of the class.
Another way I think of it is that the arguments are like members of a structure that are being sent along with the RPC, and the structure members are documented individually.
That said, I don't think it would destroy the Thrift project if we went the other way.