[SQL][MINOR] correct the comment of SortBasedAggregationIterator.safeProj

## What changes were proposed in this pull request?

This comment went stale long time ago, this PR fixes it according to my understanding.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #15095 from cloud-fan/update-comment.

sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/SortBasedAggregationIterator.scala

@@ -86,8 +86,15 @@ class SortBasedAggregationIterator(
   // The aggregation buffer used by the sort-based aggregation.
   private[this] val sortBasedAggregationBuffer: MutableRow = newBuffer
 
-  // A SafeProjection to turn UnsafeRow into GenericInternalRow, because UnsafeRow can't be
-  // compared to MutableRow (aggregation buffer) directly.
+  // This safe projection is used to turn the input row into safe row. This is necessary
+  // because the input row may be produced by unsafe projection in child operator and all the
+  // produced rows share one byte array. However, when we update the aggregate buffer according to
+  // the input row, we may cache some values from input row, e.g. `Max` will keep the max value from
+  // input row via MutableProjection, `CollectList` will keep all values in an array via
+  // ImperativeAggregate framework. These values may get changed unexpectedly if the underlying
+  // unsafe projection update the shared byte array. By applying a safe projection to the input row,
+  // we can cut down the connection from input row to the shared byte array, and thus it's safe to
+  // cache values from input row while updating the aggregation buffer.
   private[this] val safeProj: Projection = FromUnsafeProjection(valueAttributes.map(_.dataType))
 
   protected def initialize(): Unit = {