[MINOR][DOC][SQL] use one line for annotation arg value

## What changes were proposed in this pull request?

Put annotation args in one line, or API doc generation will fail.

~~~
[error] /Users/meng/src/spark/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala:1559: annotation argument needs to be a constant; found: "_FUNC_(expr) - Returns the character length of string data or number of bytes of ".+("binary data. The length of string data includes the trailing spaces. The length of binary ").+("data includes binary zeros.")
[error]     "binary data. The length of string data includes the trailing spaces. The length of binary " +
[error]                                                                                                  ^
[info] No documentation generated with unsuccessful compiler run
[error] one error found
[error] (catalyst/compile:doc) Scaladoc generation failed
[error] Total time: 27 s, completed Aug 17, 2018 3:20:08 PM
~~~

## How was this patch tested?

sbt catalyst/compile:doc passed

Closes apache#22137 from mengxr/minor-doc-fix.

Authored-by: Xiangrui Meng <meng@databricks.com>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

Author: mengxr

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

@@ -1554,10 +1554,9 @@ case class Left(str: Expression, len: Expression, child: Expression) extends Run
  * A function that returns the char length of the given string expression or
  * number of bytes of the given binary expression.
  */
+// scalastyle:off line.size.limit
 @ExpressionDescription(
-  usage = "_FUNC_(expr) - Returns the character length of string data or number of bytes of " +
-    "binary data. The length of string data includes the trailing spaces. The length of binary " +
-    "data includes binary zeros.",
+  usage = "_FUNC_(expr) - Returns the character length of string data or number of bytes of binary data. The length of string data includes the trailing spaces. The length of binary data includes binary zeros.",
   examples = """
     Examples:
       > SELECT _FUNC_('Spark SQL ');
@@ -1567,6 +1566,7 @@ case class Left(str: Expression, len: Expression, child: Expression) extends Run
       > SELECT CHARACTER_LENGTH('Spark SQL ');
        10
   """)
+// scalastyle:on line.size.limit
 case class Length(child: Expression) extends UnaryExpression with ImplicitCastInputTypes {
   override def dataType: DataType = IntegerType
   override def inputTypes: Seq[AbstractDataType] = Seq(TypeCollection(StringType, BinaryType))