[SQL] Fix docs of SQLContext.tables

Author: Yin Huai <yhuai@databricks.com>

Closes #4579 from yhuai/tablesDoc and squashes the following commits:

7f8964c [Yin Huai] Fix doc.

sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala

@@ -775,8 +775,8 @@ class SQLContext(@transient val sparkContext: SparkContext)
     DataFrame(this, catalog.lookupRelation(Seq(tableName)))
 
   /**
-   * Returns a [[DataFrame]] containing names of existing tables in the given database.
-   * The returned DataFrame has two columns, tableName and isTemporary (a column with BooleanType
+   * Returns a [[DataFrame]] containing names of existing tables in the current database.
+   * The returned DataFrame has two columns, tableName and isTemporary (a Boolean
    * indicating if a table is a temporary one or not).
    */
   def tables(): DataFrame = {
@@ -784,8 +784,8 @@ class SQLContext(@transient val sparkContext: SparkContext)
   }
 
   /**
-   * Returns a [[DataFrame]] containing names of existing tables in the current database.
-   * The returned DataFrame has two columns, tableName and isTemporary (a column with BooleanType
+   * Returns a [[DataFrame]] containing names of existing tables in the given database.
+   * The returned DataFrame has two columns, tableName and isTemporary (a Boolean
    * indicating if a table is a temporary one or not).
    */
   def tables(databaseName: String): DataFrame = {
@@ -793,7 +793,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
   }
 
   /**
-   * Returns an array of names of tables in the current database.
+   * Returns the names of tables in the current database as an array.
    */
   def tableNames(): Array[String] = {
     catalog.getTables(None).map {
@@ -802,7 +802,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
   }
 
   /**
-   * Returns an array of names of tables in the given database.
+   * Returns the names of tables in the given database as an array.
    */
   def tableNames(databaseName: String): Array[String] = {
     catalog.getTables(Some(databaseName)).map {