doc: document json rpc endpoints

fixes bitcoin#20246

Document both JSON-RPC endpoints, when they are active and which types
of requests they are able to service.

Adds two example curl requests, one for each endpoint.

doc/JSON-RPC-interface.md

@@ -5,6 +5,41 @@ The headless daemon `bitcoind` has the JSON-RPC API enabled by default, the GUI
 option. In the GUI it is possible to execute RPC methods in the Debug Console
 Dialog.
 
+## Endpoints
+
+There are two JSON-RPC endpoints on the server:
+
+1. `/`
+2. `/wallet/<walletname>/`
+
+### `/` endpoint
+
+This endpoint is always active.
+It can always service non-wallet requests and can service wallet requests when
+exactly one wallet is loaded.
+
+### `/wallet/<walletname>/` endpoint
+
+This endpoint is only activated when the wallet component has been compiled in.
+It can service both wallet and non-wallet requests.
+It MUST be used for wallet requests when two or more wallets are loaded.
+
+This is the endpoint used by bitcoin-cli when a `-rpcwallet=` parameter is passed in.
+
+Best practice would dictate using the `/wallet/<walletname>/` endpoint for ALL
+requests when multiple wallets are in use.
+
+### Examples
+
+```sh
+# Get block count from the / endpoint when rpcuser=alice and rpcport=38332
+$ curl --user alice --data-binary '{"jsonrpc": "1.0", "id": "0", "method": "getblockcount", "params": []}' -H 'content-type: text/plain;' localhost:38332/
+
+# Get balance from the /wallet/walletname endpoint when rpcuser=alice, rpcport=38332 and rpcwallet=desc-wallet
+$ curl --user alice --data-binary '{"jsonrpc": "1.0", "id": "0", "method": "getbalance", "params": []}' -H 'content-type: text/plain;' localhost:38332/wallet/desc-wallet
+
+```
+
 ## Parameter passing
 
 The JSON-RPC server supports both _by-position_ and _by-name_ [parameter