Updated user mapping docs when importing into a personal namespace

What does this MR do and why?

This MR updates importer documentation to say that user contributions are now assigned to the personal placeholder user when user_mapping_to_personal_namespace_owner is enabled.

References

• Related to #525342 (closed)
• Feature implementation in Bitbucket Server importer: !198010 (merged)
• Feature implementation in GitHub importer: !198624 (merged)
• Feature implementation in Gitea importer: !200716 (merged)
• UI warning to users: !201360 (merged)

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Related to #525342 (closed)