[DOC] Update mutate event processors docs for Data Prepper

[oeyh]

What do you want to do?

• [x] Request a change to existing documentation
• [ ] Add new documentation
• [ ] Report a technical problem with the documentation
• [ ] Other

Tell us about your request. Provide a summary of the request and all versions that are affected. The existing docs are missing a few new options. This readme has the latest info: https://github.com/opensearch-project/data-prepper/blob/main/data-prepper-plugins/mutate-event-processors/README.md

What other resources are available? Provide links to related issues, POCs, steps for testing, etc. Existing doc: https://opensearch.org/docs/latest/data-prepper/pipelines/configuration/processors/mutate-event/

[Naarcha-AWS]

@oeyh: Is this for Data Prepper 2.7?

[oeyh]

@Naarcha-AWS This is not for 2.7 (map_to_list processor is new for 2.7, for which I opened Issue #6132). The relevant code is released, but the docs have a gap.

Specifically,

• add_entries processor doc is missing format and value_expression options
• All mutate event processors have a *_when option (add_when, copy_when, delete_when, etc), it's missing in docs (and in Data Prepper's own readme files 🤷‍♂️ ).

[Naarcha-AWS]

@oeyh: Going to close this since this has been addressed. Feel free to reopen the issue if needed.