I think "Part I: Invoking GNU Fortran" of the gfortran documentation should have a section on "Compiling (and linking) mixed-language programs". Whether this should be a section or a subsection I don't know, but it's really a commonly asked question and we ought to have explanations and examples of that in the doc.
I've posted explanations and examples on comp.lang.fortran that might be used as a starting point: http://groups.google.com/group/comp.lang.fortran/msg/a62f1aa0c1592838
Linking to C++ and other languages might also be interesting. Also, when documentation about our calling conventions is written, we can link these two parts.
PS: Maybe we should also add information on how to create shared libraries of gfortran-compiled Fortran code? Or is that too generic to be in our doc?