Update documentation for forcebuy query

docs/telegram-usage.md

@@ -243,10 +243,14 @@ Return a summary of your profit/loss and performance.
 
 > **BITTREX:** Selling BTC/LTC with limit `0.01650000 (profit: ~-4.07%, -0.00008168)`
 
-### /forcebuy <pair>
+### /forcebuy <pair> [rate]
 
 > **BITTREX:** Buying ETH/BTC with limit `0.03400000` (`1.000000 ETH`, `225.290 USD`)
 
+Omitting the pair will open a query asking for the pair to buy (based on the current whitelist).
+
+![Telegram force-buy screenshot](assets/telegram_forcebuy.png)
+
 Note that for this to work, `forcebuy_enable` needs to be set to true.
 
 [More details](configuration.md#understand-forcebuy_enable)
@@ -254,7 +258,8 @@ Note that for this to work, `forcebuy_enable` needs to be set to true.
 ### /performance
 
 Return the performance of each crypto-currency the bot has sold.
-> Performance:
+
+> Performance:  
 > 1. `RCN/BTC 57.77%`  
 > 2. `PAY/BTC 56.91%`  
 > 3. `VIB/BTC 47.07%`