Document that the value of -c must be a 16-bit type (network byte order) followed by a client-ID value.
For example, to use a UUID based client-ID (type 4, RFC 6355) one could use the following cmdline option:
-c0004<128_bit_uuid_in_hex>
" -F Force IPv6-Prefix\n"
" -V <class> Set vendor-class option (base-16 encoded)\n"
" -u <user-class> Set user-class option string\n"
- " -c <clientid> Override client-ID (base-16 encoded)\n"
+ " -c <clientid> Override client-ID (base-16 encoded 16-bit type + value)\n"
" -i <iface-id> Use a custom interface identifier for RA handling\n"
" -r <options> Options to be requested (comma-separated)\n"
" -R Do not request any options except those specified with -r\n"