[SPARK-20858][DOC][MINOR] Document ListenerBus event queue size

## What changes were proposed in this pull request?

This change adds a new configuration option `spark.scheduler.listenerbus.eventqueue.size` to the configuration docs to specify the capacity of the spark listener bus event queue. Default value is 10000.

This is doc PR for [SPARK-15703](https://issues.apache.org/jira/browse/SPARK-15703).

I added option to the `Scheduling` section, however it might be more related to `Spark UI` section.

## How was this patch tested?

Manually verified correct rendering of configuration option.

Author: sadikovi <ivan.sadikov@lincolnuni.ac.nz>
Author: Ivan Sadikov <ivan.sadikov@team.telstra.com>

Closes #18476 from sadikovi/SPARK-20858.