avconv: Make the private options discovery more manifest

avconv_opt.c

@@ -2121,6 +2121,7 @@ void show_help_default(const char *opt, const char *arg)
            "    -h      -- print basic options\n"
            "    -h long -- print more options\n"
            "    -h full -- print all options (including all format and codec specific options, very long)\n"
+           "    -h type=name -- print all options for the named decoder/encoder/demuxer/muxer/filter\n"
            "    See man %s for detailed description of the options.\n"
            "\n", program_name);