[SPARK-10581] [DOCS] Groups are not resolved in scaladoc in sql classes

Groups are not resolved properly in scaladoc in following classes:

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

Author: Pravin Gadakh <pravingadakh177@gmail.com>

Closes #9148 from pravingadakh/master.