keep_original is not documented

[Randy-312]

I don't see the keep_original flag, noted in several issues 9172 and 8083.

As this is not documented for the docker input or container, I'd like to verify I can use it.

Seems like a simple documentation update miss, so hope you can correct it shortly.

[jsoriano]

@Randy-312 thanks for taking the time for opening this issue, but the keep_original flag was not merged at the end as you can see in this comment #9172 (comment)

Instead, the copy_fields processor can be used:

processors:
- copy_fields:
    fields:
      - from: message
        to: event.original
    fail_on_error: false
    ignore_missing: true

[endersonmaia]

copy_fields is not documented either #12633