Clarification of list repository tree API call

What does this MR do?

This MR adds an extra sentence in the documentation of the "List repository tree" API call, describing in slightly more detail the result returned by the call.

Are there points in the code the reviewer needs to double check?

No code changes.

Why was this MR needed?

The previous version of the documentation did perhaps not provide enough detail, leading to some confusion as exhibited in Issue #26136 (closed).

Screenshots (if relevant)

Not relevant.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Issue #26136 (closed)

Closes #26136 (closed)