improved a comment

Author: radek

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

@@ -576,7 +576,7 @@ class DataFrame private[sql](
   def as(alias: Symbol): DataFrame = as(alias.name)
 
   /**
-   * Selects a set of expressions.
+   * Selects a set of column based expressions.
    * {{{
    *   df.select($"colA", $"colB" + 1)
    * }}}