[SPARK-8462] [DOCS] Documentation fixes for Spark SQL

This fixes various minor documentation issues on the Spark SQL page

Author: Lars Francke <lars.francke@gmail.com>

Closes #6890 from lfrancke/SPARK-8462 and squashes the following commits:

dd7e302 [Lars Francke] Merge branch 'master' into SPARK-8462
34eff2c [Lars Francke] Minor documentation fixes