crtools: describe the inherit-fd option

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

crtools.c

@@ -781,6 +781,13 @@ usage:
 "  --external RES        dump objects from this list as external resources:\n"
 "                        Formats of RES:\n"
 "                            tty[rdev:dev]\n"
+"  --inherit-fd fd[<num>]:<existing>\n"
+"                        Inherit file descriptors. This allows to treat file descriptor\n"
+"                        <num> as being already opened via <existing> one and instead of\n"
+"                        trying to open we inherit it:\n"
+"                            tty[rdev:dev]\n"
+"                            pipe[inode]\n"
+"                            socket[inode]\n"
 "\n"
 "* Logging:\n"
 "  -o|--log-file FILE    log file name\n"