The only information that is exposed to APIs are the names of the RPCs, this excludes -
- the actual description of the method (optional, but useful in some case)
- the actual input parameters and their types
- the actual return type
So in this case the API is correct - it can see that there is a methods, but if has no idea how to "decorate" it since it has no information about the input and output parameters. (This is unlike anything exposed by the runtime, e.g. state/extrinsics/errors since with metadata v14 we have all the pertinent information on these, including the types)
You would need to manually fully describe the methods and add it to the API initialization options
For the apps UI specifically, this is specified in the apps-config package on a per-chain/runtime basis.