libx264: Include hint for possible values for configuring libx264

The output of the x264 commandline tool contains the most uptodate
description of the possible values for the -preset, -tune and -profile
options.

libavcodec/libx264.c

@@ -362,10 +362,10 @@ static av_cold int X264_init(AVCodecContext *avctx)
 #define OFFSET(x) offsetof(X264Context, x)
 #define VE AV_OPT_FLAG_VIDEO_PARAM | AV_OPT_FLAG_ENCODING_PARAM
 static const AVOption options[] = {
-    { "preset",        "Set the encoding preset",                    OFFSET(preset),        FF_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
-    { "tune",          "Tune the encoding params",                   OFFSET(tune),          FF_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
-    { "profile",       "Set profile restrictions",                   OFFSET(profile),       FF_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
-    { "fastfirstpass", "Use fast settings when encoding first pass", OFFSET(fastfirstpass), FF_OPT_TYPE_INT,    { 1 }, 0, 1, VE},
+    { "preset",        "Set the encoding preset (cf. x264 --fullhelp)",   OFFSET(preset),        FF_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
+    { "tune",          "Tune the encoding params (cf. x264 --fullhelp)",  OFFSET(tune),          FF_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
+    { "profile",       "Set profile restrictions (cf. x264 --fullhelp) ", OFFSET(profile),       FF_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
+    { "fastfirstpass", "Use fast settings when encoding first pass",      OFFSET(fastfirstpass), FF_OPT_TYPE_INT,    { 1 }, 0, 1, VE},
     { NULL },
 };