This interface is needed if one wants pass an array descriptor to C; other vendors have such a documentation as well. It should come with a note that the interface may change between versions.
Other compilers have this as well, e.g,
If not for the gfortran documentation, one could add it to gfortran-internal.
PR34742 is also related to documentation of our ABI/calling conventions (whatever the correct term is). I agree we should document them, as well as some other gfortran-specific non-portable stuff like "how do kinds related to integer and floating-point types", and maybe a section expanding on the -ff2c conventions. I think all this would go into a new top-level section of the documentation.