Add commenting around JSON error codes

RPC_INVALID_REQUEST and RPC_METHOD_NOT_FOUND are mapped internally to
HTTP error codes and should not be used for application-layer errors.
This commit adds commenting around those definitions to warn not to use
them for application errors.

src/rpc/protocol.h

@@ -31,9 +31,15 @@ enum HTTPStatusCode
 enum RPCErrorCode
 {
     //! Standard JSON-RPC 2.0 errors
+    // RPC_INVALID_REQUEST is internally mapped to HTTP_BAD_REQUEST (400).
+    // It should not be used for application-layer errors.
     RPC_INVALID_REQUEST  = -32600,
+    // RPC_METHOD_NOT_FOUND is internally mapped to HTTP_NOT_FOUND (404).
+    // It should not be used for application-layer errors.
     RPC_METHOD_NOT_FOUND = -32601,
     RPC_INVALID_PARAMS   = -32602,
+    // RPC_INTERNAL_ERROR should only be used for genuine errors in bitcoind
+    // (for exampled datadir corruption).
     RPC_INTERNAL_ERROR   = -32603,
     RPC_PARSE_ERROR      = -32700,