[SQL][Minor] correct some comments

Author: Sheng, Li <OopsOutOfMemory@users.noreply.github.com>
Author: OopsOutOfMemory <victorshengli@126.com>

Closes #4508 from OopsOutOfMemory/cmt and squashes the following commits:

d8a68c6 [Sheng, Li] Update ddl.scala
f24aeaf [OopsOutOfMemory] correct style

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

@@ -141,7 +141,7 @@ private[sql] class DDLParser extends AbstractSparkSQLParser with Logging {
 
   /*
    * describe [extended] table avroTable
-   * This will display all columns of table `avroTable` includes column_name,column_type,nullable
+   * This will display all columns of table `avroTable` includes column_name,column_type,comment
    */
   protected lazy val describeTable: Parser[LogicalPlan] =
     (DESCRIBE ~> opt(EXTENDED)) ~ (ident <~ ".").? ~ ident  ^^ {