Display multiple REST mappings if present

[guggero]

We use multiple REST mappings in some situations to make developer usability a bit better by circumventing some of the disadvantages of the grpc-gateway library.
Examples:

• tapd universe RPC: https://github.com/lightninglabs/taproot-assets/blob/main/taprpc/universerpc/universe.yaml#L10
• lnd lightning RPC: https://github.com/lightningnetwork/lnd/blob/c26917ee1fcf7975a0d58a2ebb02f438217dd3f6/lnrpc/lightning.yaml#L112

These don't currently show up in the API docs.