[docs] Fix outdated comment in tuning guide

When you use the SPARK_JAVA_OPTS env variable, Spark complains:

```
SPARK_JAVA_OPTS was detected (set to ' -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps ').
This is deprecated in Spark 1.0+.

Please instead use:
 - ./spark-submit with conf/spark-defaults.conf to set defaults for an application
 - ./spark-submit with --driver-java-options to set -X options for a driver
 - spark.executor.extraJavaOptions to set -X options for executors
 - SPARK_DAEMON_JAVA_OPTS to set java options for standalone daemons (master or worker)
```

This updates the docs to redirect the user to the relevant part of the configuration docs.

CC: @mengxr  but please CC someone else as needed