[Entity Analytics] Watchlist docs should mention that index pattern needs `@timestamp` field.

[ymao1]

In the Watchlist docs, we should mention that for Rule Based Data Sources > Index Pattern, the selected index pattern must have the @timestamp field. Selecting an index pattern without a @timestamp field will cause no entities ever to be synced. On the Kibana side, we're adding validation to that field in the UI in elastic/kibana#287385

This has been a requirement since Watchlists were added in 9.4

[github-actions]

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