Document bulk_max_size for all outputs, not just Elasticsearch

[dedemorton]

The doc about configuration options currently describes the bulk_max_size option for Elasticsearch output, but does not contain descriptions of this option for other output types (Logstash, console, and file). Need to add these descriptions to the documentation.

Here are some comments by @urso carried over from PR #568

So we've got multiple output plugins:

Elasticsearch
logstash
console
file

The default bulk_max_size is used for all output plugins but elasticsearch, which sets the default to 50.

This options sets maximum number of events that can be combined internally into batches and will be publishable by the output plugins =>

if beat tries to send single events, the events are collected into batches

if beat tries to publish large batch of event (bigger bulk_max_size), the batch will be split.

Bigger batch sizes can improve performance due to ammortizing per event sending overhead. On the other hand to big batch sizes can increase processing time such that queues in logstash/elasticsearch can not be processed -> APIs return errors, connections get killed or publish requests time out. This increases latency and lowers throughput for indexing events.