[SPARK] Improve documentation in DataSkippingReader (delta-io#4323)

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#### Which Delta project/connector is this regarding?
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- [x] Spark
- [ ] Standalone
- [ ] Flink
- [ ] Kernel
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## Description

A minor improvement to documentation in `DataSkippingReader`. 

## How was this patch tested?

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N/A. Documentation-only.

## Does this PR introduce _any_ user-facing changes?

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No.

Author: ala

spark/src/main/scala/org/apache/spark/sql/delta/stats/DataSkippingReader.scala

@@ -421,6 +421,10 @@ trait DataSkippingReaderBase
      * NOTE: The skipping predicate does *NOT* need to worry about missing stats columns (which also
      * manifest as NULL). That case is handled separately by `verifyStatsForFilter` (which disables
      * skipping for any file that lacks the needed stats columns).
+     *
+     * @return An optional data skipping predicate, if this function returns None, then this means
+     * that the dataFilter Expression is not eligible for data skipping, i.e. we cannot skip any
+     * files.
      */
     private[stats] def constructDataFilters(dataFilter: Expression):
         Option[DataSkippingPredicate] = dataFilter match {