Update docs for retarget MRs

What does this MR do?

Update docs for retarget MRs

This ensures that:

• it is ready for production use
• documents remove source branch caveat

This is a follow-up for:

• !53710 (merged)
• #321559
• !56233 (merged)
• gitlab-com/www-gitlab-com!74726 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)