openmpi: explain why pmix is not found with pkg-config

pkgs/by-name/op/openmpi/package.nix

@@ -114,6 +114,8 @@ stdenv.mkDerivation (finalAttrs: {
     (lib.enableFeature cudaSupport "mca-dso")
     (lib.enableFeature fortranSupport "mpi-fortran")
     (lib.withFeatureAs stdenv.hostPlatform.isLinux "libnl" (lib.getDev libnl))
+    # From some reason, without this the darwin build fails with cyclic
+    # references between $dev and $out
     "--with-pmix=${lib.getDev pmix}"
     "--with-pmix-libdir=${lib.getLib pmix}/lib"
     # Puts a "default OMPI_PRTERUN" value to mpirun / mpiexec executables