[SPARK-9814][SQL] Fix comments of EqualNullSafe()

Author: HyukjinKwon

sql/core/src/main/scala/org/apache/spark/sql/sources/filters.scala

@@ -37,8 +37,9 @@ abstract class Filter
 case class EqualTo(attribute: String, value: Any) extends Filter
 
 /**
- * A filter that evaluates to `true` iff the attribute evaluates to a value
- * null safe and equal to `value`.
+ * Performs equality comparison, similar to [[EqualTo]]. However, this differs from [[EqualTo]] 
+ * in that it returns `true` (rather than NULL) if both inputs are NULL, and `false` 
+ * (rather than NULL) if one of the input is NULL and the other is not NULL.
  *
  * @since 1.5.0
  */