Update doc/user/project/repository/index.md

What does this MR do?

Update the docs to be more explicit on when the details will appear.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/48647#note_104021317

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides