[docs] Fix outdated comment in tuning guide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

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #3592 from jkbradley/tuning-doc and squashes the following commits:

0760ce1 [Joseph K. Bradley] fixed outdated comment in tuning guide

docs/tuning.md

@@ -143,8 +143,7 @@ the space allocated to the RDD cache to mitigate this.
 **Measuring the Impact of GC**
 
 The first step in GC tuning is to collect statistics on how frequently garbage collection occurs and the amount of
-time spent GC. This can be done by adding `-verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps` to your
-`SPARK_JAVA_OPTS` environment variable. Next time your Spark job is run, you will see messages printed in the worker's logs
+time spent GC. This can be done by adding `-verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps` to the Java options.  (See the [configuration guide](configuration.html#Dynamically-Loading-Spark-Properties) for info on passing Java options to Spark jobs.)  Next time your Spark job is run, you will see messages printed in the worker's logs
 each time a garbage collection occurs. Note these logs will be on your cluster's worker nodes (in the `stdout` files in
 their work directories), *not* on your driver program.