Better oneof documentation for request parameters

[dazuma]

The string added to oneof fields in proto_docs (that notes the field is part of a mutually exclusive set, etc.) also appears in the parameter documentation for RPC methods if the request message includes a oneof. The wording in that context is a bit confusing. We should still have some kind of note, but it should refer to the fields as "parameters" and should note that at most one of them can be set, and if multiple ones are set, only one will take effect, and it's not defined which one.