Document Audit Events features are single-level

What does this MR do?

Audit Events features only display the events from the given level. For example, the group-level interface only displays group-level events, not any project-level events that may have occurred within that group. This adds documentation for both the API and the Audit Events pages to explain this nuance.

This nuance was documented in the group-level interface itself in !36238 (merged)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.