[SPARK-23736][SQL] Fixing failing unit test from DDLSuite.

Author: mn-mikke

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/unresolved.scala

@@ -485,6 +485,15 @@ case class UnresolvedOrdinal(ordinal: Int)
  * Concatenates multiple columns of the same type into one.
  * @param children Could be string, binary or array expressions
  */
+@ExpressionDescription(
+  usage = "_FUNC_(col1, col2, ..., colN) - Returns the concatenation of col1, col2, ..., colN.",
+  examples = """
+    Examples:
+      > SELECT _FUNC_('Spark', 'SQL');
+       SparkSQL
+      > SELECT _FUNC_(array(1, 2, 3), array(4, 5), array(6));
+       [1,2,3,4,5,6]
+  """)
 case class UnresolvedConcat(children: Seq[Expression]) extends Expression
   with Unevaluable {
   override def dataType: DataType = throw new UnresolvedException(this, "dataType")

sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala

@@ -1664,10 +1664,10 @@ abstract class DDLSuite extends QueryTest with SQLTestUtils {
     // STRING
     checkAnswer(
       sql("DESCRIBE FUNCTION 'concat'"),
-      Row("Class: org.apache.spark.sql.catalyst.expressions.Concat") ::
+      Row("Class: org.apache.spark.sql.catalyst.analysis.UnresolvedConcat") ::
         Row("Function: concat") ::
-        Row("Usage: concat(str1, str2, ..., strN) - " +
-            "Returns the concatenation of str1, str2, ..., strN.") :: Nil
+        Row("Usage: concat(col1, col2, ..., colN) - " +
+            "Returns the concatenation of col1, col2, ..., colN.") :: Nil
     )
     // extended mode
     checkAnswer(