Update docs: Transferring projects with container images

In the documentation for transferring projects, we mention that the project must not contain container images. But now, with Allow transferring of projects with container r... (#499163 - closed), projects with container images can be transferred within the same top-level namespace as long as they have the metadata database enabled, this is true for GitLab.com and self-managed who have migrated.

For this issue, we want to update the documentation to reflect these changes.