Expand docs for impersonated AuditEvents

What does this MR do?

This impersonation data documentation was considered "brief to the point of being unclear" (source). This MR expands it with helpful details. This also removes the tier badge in favor of a link to the new impersonation documentation, because impersonation audit events are entirely dependent on impersonation and we don't want competing sources of truth about which tier impersonation exists at.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team