Fix kernel-doc processing of SYSCALL wrappers.

The SYSCALL wrapper patches played havoc with kernel-doc for
syscalls.  Syscalls that were scanned for DocBook processing
reported warnings like this one, for sys_tgkill:

Warning(kernel/signal.c:2285): No description found for parameter 'tgkill'
Warning(kernel/signal.c:2285): No description found for parameter 'pid_t'
Warning(kernel/signal.c:2285): No description found for parameter 'int'

because the macro parameters all "look like" function parameters,
although they are not:

/**
 *  sys_tgkill - send signal to one specific thread
 *  @tgid: the thread group ID of the thread
 *  @pid: the PID of the thread
 *  @sig: signal to be sent
 *
 *  This syscall also checks the @tgid and returns -ESRCH even if the PID
 *  exists but it's not belonging to the target process anymore. This
 *  method solves the problem of threads exiting and PIDs getting reused.
 */
SYSCALL_DEFINE3(tgkill, pid_t, tgid, pid_t, pid, int, sig)
...