Make settxfee clear that units are btc/kb.

src/rpcblockchain.cpp

@@ -108,8 +108,8 @@ Value settxfee(const Array& params, bool fHelp)
 {
     if (fHelp || params.size() < 1 || params.size() > 1)
         throw runtime_error(
-            "settxfee <amount>\n"
-            "<amount> is a real and is rounded to the nearest 0.00000001");
+            "settxfee <amount btc/kb>\n"
+            "<amount> is a real and is rounded to the nearest 0.00000001 btc per kb");
 
     // Amount
     int64 nAmount = 0;