## What changes were proposed in this pull request?

Like `SPARK_JAVA_OPTS` and `SPARK_CLASSPATH`, we will remove the document for `SPARK_WORKER_INSTANCES` to discourage user not to use them. If they are actually used, SparkConf will show a warning message as before.

## How was this patch tested?

Manually tested.

Author: bomeng <bmeng@us.ibm.com>

Closes #13533 from bomeng/SPARK-15781.