docs: document datetime rebasing and V2 API limitations for DataFusion-based scans (apache#3259)

Add two new limitations to the shared limitations section for
native_datafusion and native_iceberg_compat scan implementations:

1. No support for datetime rebasing detection or the
   spark.comet.exceptionOnDatetimeRebase configuration. When reading
   Parquet files with dates/timestamps written before Spark 3.0
   (hybrid Julian/Gregorian calendar), these implementations cannot
   detect legacy values and may produce incorrect results for dates
   before October 15, 1582.

2. No support for Spark's Datasource V2 API. When V2 is enabled,
   Comet falls back to native_comet.

Co-authored-by: Claude Opus 4.5 <[email protected]>

Author: andygrove

docs/source/contributor-guide/parquet_scans.md

@@ -50,6 +50,15 @@ The `native_datafusion` and `native_iceberg_compat` scans share the following li
   `spark.comet.scan.unsignedSmallIntSafetyCheck=false`. Note that `ByteType` columns are always safe because they can
   only come from signed `INT8`, where truncation preserves the signed value.
 - No support for default values that are nested types (e.g., maps, arrays, structs). Literal default values are supported.
+- No support for datetime rebasing detection or the `spark.comet.exceptionOnDatetimeRebase` configuration. When reading
+  Parquet files containing dates or timestamps written before Spark 3.0 (which used a hybrid Julian/Gregorian calendar),
+  the `native_comet` implementation can detect these legacy values and either throw an exception or read them without
+  rebasing. The DataFusion-based implementations do not have this detection capability and will read all dates/timestamps
+  as if they were written using the Proleptic Gregorian calendar. This may produce incorrect results for dates before
+  October 15, 1582.
+- No support for Spark's Datasource V2 API. When `spark.sql.sources.useV1SourceList` does not include `parquet`,
+  Spark uses the V2 API for Parquet scans. The DataFusion-based implementations only support the V1 API, so Comet
+  will fall back to `native_comet` when V2 is enabled.
 
 The `native_datafusion` scan has some additional limitations: