Clarify direct transfer of only projects available via the API

What does this MR do?

At this current point in time, direct transfer in the UI only works within the context of importing whole groups. You can import projects by importing the whole group that they belong to, but you cannot import only specific projects within a group. If you wish to import only specific projects rather than entire groups with direct transfer, it's possible to do so via the API. Pointing this out within this part of the documentation makes it so that users know this is the case, without having to have seen the API documentation first.

Related issues

• #390515 (closed)

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