Merge bitcoin#27225: doc: document json rpc endpoints

65e3abc doc: document json rpc endpoints (willcl-ark)

Pull request description:

  fixes bitcoin#20246

  This documents the two JSON-RPC endpoints available, details when they are active, specifies when they can or must be used, and outlines some known behaviour quirks.

ACKs for top commit:
  fanquake:
    ACK 65e3abc - Seems fine. Can be improved if need be.

Tree-SHA512: d557c2caf000a1bdd7b46c9da38afe63dc28446ba4a961526f1af3cec81d994a9da663e02c81ebdc4f609b794585349cfca77a582dc1e788c120de1d3b4c7db6

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