update docs

docs/source/user-guide/installation.md

@@ -67,7 +67,8 @@ $SPARK_HOME/bin/spark-shell \
     --conf spark.sql.extensions=org.apache.comet.CometSparkSessionExtensions \
     --conf spark.comet.enabled=true \
     --conf spark.comet.exec.enabled=true \
-    --conf spark.comet.exec.all.enabled=true
+    --conf spark.comet.exec.all.enabled=true \
+    --conf spark.comet.explainFallback.enabled=true
 ```
 
 ### Verify Comet enabled for Spark SQL query
@@ -95,6 +96,16 @@ INFO src/lib.rs: Comet native library initialized
              PushedFilters: [IsNotNull(a), GreaterThan(a,5)], ReadSchema: struct<a:int>
 ```
 
+With the configuration `spark.comet.explainFallback.enabled=true`, Comet will log any reasons that prevent a plan from
+being executed natively.
+
+```scala
+scala> spark.sql("select * from t1 where cast(cast(a as double) as decimal(10,2)) > 5").show
+WARN CometSparkSessionExtensions$CometExecRule: Comet cannot execute this plan natively because:
+	- Comet does not guarantee correct results for cast from DoubleType to DecimalType(10,2) with timezone Some(America/Denver) and evalMode LEGACY (No overflow check). To enable all incompatible casts, set spark.comet.cast.allowIncompatible=true
+	- CollectLimit is not supported
+```
+
 ### Enable Comet shuffle
 
 Comet shuffle feature is disabled by default. To enable it, please add related configs: