Review and update development docs for Direct Transfer
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=519937) </details> <!--IssueSummary end--> We've made several improvements to Direct Transfer lately, including the release of Improved User Contribution Mapping. Let's review the development docs under https://docs.gitlab.com/development/bulk_import/ to ensure docs are up-to-date
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