[Docs] Document SharePoint Online expand_site_group_members setting (8.19)

[Jan-Kazlouski-elastic]

Backport of documentation for expand_site_group_members to 8.19.

Documents the expand_site_group_members configuration field from elastic/connectors#4396 and elastic/kibana#288070.

Part of elastic/chat-program#47.

Because 8.19 uses legacy asciidoc docs (with separate managed/native and self-managed configuration sections), the field is documented in both sections of connectors-sharepoint-online.asciidoc:

• Managed (native) section: Expand site group members UI toggle
• Self-managed section: expand_site_group_members config key

Both sections also describe compact site-group DLS behavior in Document-level security. Introduced in 8.19.22.

Docs-only change; no code or console snippets affected.

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