doc: Add external interface consistency guarantees

doc/REST-interface.md

@@ -6,6 +6,12 @@ The REST API can be enabled with the `-rest` option.
 The interface runs on the same port as the JSON-RPC interface, by default port 8332 for mainnet, port 18332 for testnet,
 and port 18443 for regtest.
 
+REST Interface consistency guarantees
+-------------------------------------
+
+The [same guarantees as for the RPC Interface](/doc/JSON-RPC-interface.md#rpc-consistency-guarantees)
+apply.
+
 Supported API
 -------------