Document rescanblockchain returned stop_height being null

Suggested bitcoin/bitcoin#14711 (comment)

Author: ryanofsky

src/wallet/rpcwallet.cpp

@@ -3412,8 +3412,8 @@ UniValue rescanblockchain(const JSONRPCRequest& request)
                 },
                 RPCResult{
             "{\n"
-            "  \"start_height\"     (numeric) The block height where the rescan has started.\n"
-            "  \"stop_height\"      (numeric) The height of the last rescanned block.\n"
+            "  \"start_height\"     (numeric) The block height where the rescan started (the requested height or 0)\n"
+            "  \"stop_height\"      (numeric) The height of the last rescanned block. May be null in rare cases if there was a reorg and the call didn't scan any blocks because they were already scanned in the background.\n"
             "}\n"
                 },
                 RPCExamples{