Clarifies documentation for user, group & project transfers

What does this MR do?

Clarifies documentation for user, group & project transfers introduced in https://gitlab.com/gitlab-org/gitlab-ce/issues/35385.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered