[Ingest pipelines] Ability to keep fields instead of remove fields in processors

[florent-leborgne]

Version: 9.2
PR: elastic/kibana#225638

That's maybe an @elastic/ingest-docs matter, or maybe not, just tagging in case it is.

[florent-leborgne]

After checking the docs, it just needs a small note that "fields" isn't required when using "keep". We don't seem to have UI specific docs for processors, so this is just a super small edit needed as keep already existed on the elasticsearch side. It was just not present in the UI (for which we don't have specific per-processor docs)