docs: document negative zero cast-to-string incompatibility (#3811)

Author: andygrove

docs/source/user-guide/latest/compatibility.md

@@ -153,6 +153,13 @@ Cast operations in Comet fall into three levels of support:
   Spark.
 - **N/A**: Spark does not support this cast.
 
+### Negative Zero
+
+When casting floating-point values to strings, Spark normalizes negative zero (`-0.0`) to `"0.0"`, but Comet
+may produce `"-0.0"`. Since negative zero and positive zero are semantically equivalent (`-0.0 == 0.0` is true
+in IEEE 754), this difference is unlikely to affect real-world results. See
+[#1036](https://github.com/apache/datafusion-comet/issues/1036) for more details.
+
 ### Legacy Mode
 
 <!--BEGIN:CAST_LEGACY_TABLE-->

spark/src/main/scala/org/apache/comet/expressions/CometCast.scala

@@ -149,6 +149,7 @@ object CometCast extends CometExpressionSerde[Cast] with CometExprShim {
         isSupported(dt.elementType, dt1.elementType, timeZoneId, evalMode)
       case (dt: DataType, _) if dt.typeName == "timestamp_ntz" =>
         // https://github.com/apache/datafusion-comet/issues/378
+        // https://github.com/apache/datafusion-comet/issues/3179
         toType match {
           case DataTypes.TimestampType | DataTypes.DateType | DataTypes.StringType =>
             Incompatible()

spark/src/main/scala/org/apache/comet/serde/arrays.scala

@@ -247,7 +247,12 @@ object CometArrayMin extends CometExpressionSerde[ArrayMin] {
 
 object CometArraysOverlap extends CometExpressionSerde[ArraysOverlap] {
 
-  override def getSupportLevel(expr: ArraysOverlap): SupportLevel = Incompatible(None)
+  override def getSupportLevel(expr: ArraysOverlap): SupportLevel =
+    Incompatible(
+      Some(
+        "Inconsistent behavior with NULL values" +
+          " (https://github.com/apache/datafusion-comet/issues/3645)" +
+          " (https://github.com/apache/datafusion-comet/issues/2036)"))
 
   override def convert(
       expr: ArraysOverlap,
@@ -446,7 +451,11 @@ object CometArrayInsert extends CometExpressionSerde[ArrayInsert] {
 
 object CometArrayUnion extends CometExpressionSerde[ArrayUnion] {
 
-  override def getSupportLevel(expr: ArrayUnion): SupportLevel = Incompatible(None)
+  override def getSupportLevel(expr: ArrayUnion): SupportLevel =
+    Incompatible(
+      Some(
+        "Correctness issue" +
+          " (https://github.com/apache/datafusion-comet/issues/3644)"))
 
   override def convert(
       expr: ArrayUnion,