[Bug] RBAC for Assistant (#2469)

Fixes /elastic/security-docs/issues/6769 by updating outdated
information about RBAC for AI Assistant. The previous version showed
only 4 security privileges that the user could update, the current total
is 8.

The updated image appears:
[here](https://www.elastic.co/docs/solutions/security/ai/ai-assistant-knowledge-base#knowledge-base-rbac)

Author: benironside