Fix documentation for RPC objects/methods with no Dispatch table entry

The current implementation of rpc-docs-tool only generates documentation for objects that support at least one RPC method, and for methods that are invocable on at lease one RPC object.

This approach omits:

• methods like the version of rpc:release introduced in !2573 (merged), which applies to object IDs rather than objects.
• methods that aren't yet implemented on any objects (which is probably okay to omit)
• objects that are only returned, and don't yet have methods on them.

It would be good to clear this up, probably once !2573 (merged) is merged.