[MINOR] inaccurate comments for showString()

Author: CodingCat <zhunansjtu@gmail.com>

Closes #8050 from CodingCat/minor and squashes the following commits:

5bc4b89 [CodingCat] inaccurate comments

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

@@ -168,7 +168,7 @@ class DataFrame private[sql](
   }
 
   /**
-   * Internal API for Python
+   * Compose the string representing rows for output
    * @param _numRows Number of rows to show
    * @param truncate Whether truncate long strings and align cells right
    */