Add some compatibility around buy_tag

docs/strategy-advanced.md

@@ -77,7 +77,7 @@ class AwesomeStrategy(IStrategy):
 
 ***
 
-## Buy Tag
+## Enter Tag
 
 When your strategy has multiple buy signals, you can name the signal that triggered.
 Then you can access you buy signal on `custom_sell`
@@ -89,7 +89,7 @@ def populate_buy_trend(self, dataframe: DataFrame, metadata: dict) -> DataFrame:
             (dataframe['rsi'] < 35) &
             (dataframe['volume'] > 0)
         ),
-        ['buy', 'buy_tag']] = (1, 'buy_signal_rsi')
+        ['buy', 'enter_tag']] = (1, 'buy_signal_rsi')
 
     return dataframe
 
@@ -104,7 +104,7 @@ def custom_sell(self, pair: str, trade: Trade, current_time: datetime, current_r
 ```
 
 !!! Note
-    `buy_tag` is limited to 100 characters, remaining data will be truncated.
+    `enter_tag` is limited to 100 characters, remaining data will be truncated.
 
 ## Exit tag