chore: add bt note to _generic_performance_query

Author: xmatthias

freqtrade/persistence/trade_model.py

@@ -1922,6 +1922,7 @@ def _generic_performance_query(columns: list, filters: list, fallback: str = "")
         - profit_ratio
         - profit_sum_abs
         - count
+        NOTE: Not supported in Backtesting.
         """
         columns_coal = [func.coalesce(c, fallback).label(c.name) for c in columns]
         pair_costs = (