[SPARK-22086][DOCS] Add expression description for CASE WHEN

## What changes were proposed in this pull request?

In SQL conditional expressions, only CASE WHEN lacks for expression description. This patch fills the gap.

## How was this patch tested?

Only documentation change.

Author: Liang-Chi Hsieh <[email protected]>

Closes #19304 from viirya/casewhen-doc.

Author: viirya

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/conditionalExpressions.scala

@@ -223,7 +223,22 @@ abstract class CaseWhenBase(
  */
 // scalastyle:off line.size.limit
 @ExpressionDescription(
-  usage = "CASE WHEN expr1 THEN expr2 [WHEN expr3 THEN expr4]* [ELSE expr5] END - When `expr1` = true, returns `expr2`; when `expr3` = true, return `expr4`; else return `expr5`.")
+  usage = "CASE WHEN expr1 THEN expr2 [WHEN expr3 THEN expr4]* [ELSE expr5] END - When `expr1` = true, returns `expr2`; else when `expr3` = true, returns `expr4`; else returns `expr5`.",
+  arguments = """
+    Arguments:
+      * expr1, expr3 - the branch condition expressions should all be boolean type.
+      * expr2, expr4, expr5 - the branch value expressions and else value expression should all be
+          same type or coercible to a common type.
+  """,
+  examples = """
+    Examples:
+      > SELECT CASE WHEN 1 > 0 THEN 1 WHEN 2 > 0 THEN 2.0 ELSE 1.2 END;
+       1
+      > SELECT CASE WHEN 1 < 0 THEN 1 WHEN 2 > 0 THEN 2.0 ELSE 1.2 END;
+       2
+      > SELECT CASE WHEN 1 < 0 THEN 1 WHEN 2 < 0 THEN 2.0 ELSE null END;
+       NULL
+  """)
 // scalastyle:on line.size.limit
 case class CaseWhen(
     val branches: Seq[(Expression, Expression)],