[SPARK-13040][DOCS] Update JDBC deprecated SPARK_CLASSPATH documentation

Update JDBC documentation based on http://stackoverflow.com/a/30947090/219530 as SPARK_CLASSPATH is deprecated.

Also, that's how it worked, it didn't work with the SPARK_CLASSPATH or the --jars alone.

This would solve issue: https://issues.apache.org/jira/browse/SPARK-13040

Author: Sebastián Ramírez <tiangolo@gmail.com>

Closes #10948 from tiangolo/patch-docs-jdbc.