docs: Documentation updates for LocalTableScan and WindowExec (#2742)

Author: andygrove

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

@@ -68,6 +68,11 @@ Comet uses the Rust regexp crate for evaluating regular expressions, and this ha
 regular expression engine. Comet will fall back to Spark for patterns that are known to produce different results, but
 this can be overridden by setting `spark.comet.regexp.allowIncompatible=true`.
 
+## Window Functions
+
+Comet's support for window functions is incomplete and known to be incorrect. It is disabled by default and 
+should not be used in production. The feature will be enabled in a future release. Tracking issue: [#2721](https://github.com/apache/datafusion-comet/issues/2721).
+
 ## Cast
 
 Cast operations in Comet fall into three levels of support:

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

@@ -202,6 +202,23 @@ incompatible expressions.
 | VariancePop   |            | Yes                       |                                                                  |
 | VarianceSamp  |            | Yes                       |                                                                  |
 
+## Window Functions
+
+```{warning}
+Window support is disabled by default due to known correctness issues. Tracking issue: [#2721](https://github.com/apache/datafusion-comet/issues/2721).
+```
+
+Comet supports using the following aggregate functions within window contexts with PARTITION BY and ORDER BY clauses. 
+
+| Expression | Spark-Compatible? | Compatibility Notes                          |
+| ---------- | ----------------- | -------------------------------------------- |
+| Count      | Yes               |     |
+| Max        | Yes               |     |
+| Min        | Yes               |     |
+| Sum        | Yes               |     |
+
+**Note:** Dedicated window functions such as `rank`, `dense_rank`, `row_number`, `lag`, `lead`, `ntile`, `cume_dist`, `percent_rank`, and `nth_value` are not currently supported and will fall back to Spark.
+
 ## Array Expressions
 
 | Expression     | Spark-Compatible? | Compatibility Notes                                                                                                                                                                       |

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

@@ -23,7 +23,7 @@ The following Spark operators are currently replaced with native versions. Query
 not supported by Comet will fall back to regular Spark execution.
 
 | Operator                | Spark-Compatible? | Compatibility Notes                                                                                                |
-| ----------------------- | ----------------- | ------------------------------------------------------------------------------------------------------------------ |
+|-------------------------|-------------------|--------------------------------------------------------------------------------------------------------------------|
 | BatchScanExec           | Yes               | Supports Parquet files and Apache Iceberg Parquet scans. See the [Comet Compatibility Guide] for more information. |
 | BroadcastExchangeExec   | Yes               |                                                                                                                    |
 | BroadcastHashJoinExec   | Yes               |                                                                                                                    |
@@ -33,13 +33,14 @@ not supported by Comet will fall back to regular Spark execution.
 | GlobalLimitExec         | Yes               |                                                                                                                    |
 | HashAggregateExec       | Yes               |                                                                                                                    |
 | LocalLimitExec          | Yes               |                                                                                                                    |
+| LocalTableScanExec      | No                | Experimental and disabled by default.                                                                              |
 | ObjectHashAggregateExec | Yes               | Supports a limited number of aggregates, such as `bloom_filter_agg`.                                               |
 | ProjectExec             | Yes               |                                                                                                                    |
 | ShuffleExchangeExec     | Yes               |                                                                                                                    |
 | ShuffledHashJoinExec    | Yes               |                                                                                                                    |
 | SortExec                | Yes               |                                                                                                                    |
 | SortMergeJoinExec       | Yes               |                                                                                                                    |
 | UnionExec               | Yes               |                                                                                                                    |
-| WindowExec              | Yes               |                                                                                                                    |
+| WindowExec              | No                | Disabled by default due to known correctness issues.                                                               |
 
 [Comet Compatibility Guide]: compatibility.md