Clarify import foreign schema

Since we don't have some kind of evaluate directive for ddl, we use the import foreign schema construct in a similar manner by using the ddl-file repository type.

It's possible to split ddl for a schema into multiple file, which should be described in the docs.

It's also possible to use import foreign schema into a virtual schema, but the foreign keyword is still required. It should be made optional and described as ignored in the docs.