uhd: mention buggy usrp_hwd.py utility

pkgs/applications/radio/uhd/default.nix

···
   ] ++ optionals (enableUtils) [
     python3.pkgs.requests
     python3.pkgs.six
+
+    /* These deps are needed for the usrp_hwd.py utility, however even if they
+    would have been added here, the utility wouldn't have worked because it
+    depends on an old python library mprpc that is not supported for Python >
+    3.8. See also report upstream:
+    https://github.com/EttusResearch/uhd/issues/744
+
+    python3.pkgs.gevent
+    python3.pkgs.pyudev
+    python3.pkgs.pyroute2
+
+    */
   ];
   passthru = {
     runtimePython = python3.withPackages (ps: finalAttrs.pythonPath);