Elastic Defend advanced settings (#1445)

Resolves elastic/security-docs#2234 by
documenting the Elastic Defend policy advanced settings in the Reference
section.
The setting descriptions consist of the Kibana tooltip text (in italics)
and, for most settings, an additional description. The Kibana tooltip
text was kept because some settings don't have an additional
description.

Preview: [Elastic Defend advanced
settings](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1445/reference/security/defend-advanced-settings)

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Co-authored-by: Asuka Nakajima <[email protected]>
Co-authored-by: Arianna Laudazzi <[email protected]>
Co-authored-by: Daniel Ferullo <[email protected]>

Author: 4 people