docs: document the path and benchmark settings

.env.example

@@ -43,6 +43,12 @@ NVIDIA_API_KEY=
 #TRADINGAGENTS_MAX_RISK_ROUNDS=1
 #TRADINGAGENTS_CHECKPOINT_ENABLED=false
 
+# Paths and alpha benchmark. Unset uses ~/.tradingagents and the regional index.
+#TRADINGAGENTS_RESULTS_DIR=
+#TRADINGAGENTS_CACHE_DIR=
+#TRADINGAGENTS_MEMORY_LOG_PATH=
+#TRADINGAGENTS_BENCHMARK_TICKER=SPY
+
 # Lower temperature means less run-to-run variation on models that honor it.
 #TRADINGAGENTS_TEMPERATURE=0.0