[docs] Clarify decorate_events behavior

[ismael-hasan]

cc/ @karenzone

Currently, the documentation states:
Adds metadata about the Event Hub, including Event Hub name, consumer_group, processor_host, partition, offset, sequence, timestamp, and event_size.

It is easy to miss that metadata refers to the @metadata field in the events, and also, that this field is a special field that is not sent to the output; so, enabling decorate_events without further action does not change anything regarding what we will send from Logstash.

A clear example of this confussion can be found in #7 .

It would be good to modify the documentation to make more clear that to send these fields they must be copied from @metadata to fields in the event

[Skypex]

It spent today some time on the same "issue"...
Would appreciate it too, if you could point out that this will be moved the the "invisible" @metadata field.
Many thanks!

[cskowronnek]

+1 for adding this behavior to the documentation. I spent a significant amount of time trying to figure out why I don't see metadata field at the output.