update help message regarding vbr mode

Author: nu774

src/main.c

@@ -121,10 +121,9 @@ PROGNAME " %s\n"
 " -m, --bitrate-mode <n>        Bitrate configuration\n"
 "                                 0: CBR (default)\n"
 "                                 1-5: VBR\n"
-"                               (VBR mode is not officially supported, and\n"
-"                                works only on a certain combination of\n"
-"                                parameter settings, sample rate, and\n"
-"                                channel configuration)\n"
+"                               (VBR mode 1 is tuned for HE-AAC v2\n"
+"                                VBR mode 2 is tuned for HE-AAC\n"
+"                                VBR mode 3 or greater is for AAC LC)\n"
 " -w, --bandwidth <n>           Frequency bandwidth in Hz (AAC LC only)\n"
 " -a, --afterburner <n>         Afterburner\n"
 "                                 0: Off\n"