lavfi/testsrc: add comment in options

libavfilter/vsrc_testsrc.c

@@ -67,6 +67,8 @@ static const AVOption testsrc_options[]= {
     { "duration", "set video duration", OFFSET(duration), AV_OPT_TYPE_STRING, {.str = NULL},      0, 0 },
     { "d",        "set video duration", OFFSET(duration), AV_OPT_TYPE_STRING, {.str = NULL},      0, 0 },
     { "sar",      "set video sample aspect ratio", OFFSET(sar), AV_OPT_TYPE_RATIONAL, {.dbl= 1},  0, INT_MAX },
+
+    /* only used by testsrc */
     { "decimals", "set number of decimals to show", OFFSET(nb_decimals), AV_OPT_TYPE_INT, {.dbl=0},  INT_MIN, INT_MAX },
     { "n",        "set number of decimals to show", OFFSET(nb_decimals), AV_OPT_TYPE_INT, {.dbl=0},  INT_MIN, INT_MAX },
     { NULL },