[DOC] Add processor _when condition to Data Prepper processor documentation

[graytaylor0]

What do you want to do?

Many of the processors in Data Prepper contain conditional when parameters to conditionally run a processor on certain Events.

For example, the grok_when parameter for the grok processor is documented here (https://opensearch.org/docs/latest/data-prepper/pipelines/configuration/processors/grok/).

The following processors have when conditions, but they are not documented

• add_entries -> add_when
• aggregate -> aggregate_when
• convert_entry_type -> convert_when
• copy_values -> copy_when
• csv -> csv_when
• date -> date_when
• delete_entries -> delete_when
• list_to_map -> list_to_map_when
• split_string -> split_when
• substitute_string -> substitute_when
• rename_keys -> rename_when
• key_value -> key_value_when

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

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