[SPARK-8184][SQL] Add additional function description for weekofyear

## What changes were proposed in this pull request?

Add additional function description for weekofyear.

## How was this patch tested?

 manual tests

![weekofyear](https://cloud.githubusercontent.com/assets/5399861/26525752/08a1c278-4394-11e7-8988-7cbf82c3a999.gif)

Author: Yuming Wang <wgyumg@gmail.com>

Closes #18132 from wangyum/SPARK-8184.

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

@@ -402,13 +402,15 @@ case class DayOfMonth(child: Expression) extends UnaryExpression with ImplicitCa
   }
 }
 
+// scalastyle:off line.size.limit
 @ExpressionDescription(
-  usage = "_FUNC_(date) - Returns the week of the year of the given date.",
+  usage = "_FUNC_(date) - Returns the week of the year of the given date. A week is considered to start on a Monday and week 1 is the first week with >3 days.",
   extended = """
     Examples:
       > SELECT _FUNC_('2008-02-20');
        8
   """)
+// scalastyle:on line.size.limit
 case class WeekOfYear(child: Expression) extends UnaryExpression with ImplicitCastInputTypes {
 
   override def inputTypes: Seq[AbstractDataType] = Seq(DateType)