Document how to make interdependent targets on different namespaces

The following discussion from !1247 (merged) should be addressed:

• @simon.praetorius started a discussion: (+13 comments)

  Unfortunately, this does not work properly. If one target depends on the other target (e.g. via target_link_libraries), the dependency must be included before. Otherwise I get the fatal error "The following imported targets are referenced, but are missing..." automatically generated in the target file. Not sure how to fix this properly. I have to create the libraries in the right order, I think, and then it works. But this is not the way to want to resolve dependencies.