sysrq: fix ftrace help msg & doc.

Impact: update documentation and help messages

We have a conventional method of explicitly stating the
sysrq action key in a sysrq help message, so change
dump-ftrace-buffer to use that method and add it to
Documentation/sysrq.txt.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

drivers/char/sysrq.c

@@ -283,7 +283,7 @@ static void sysrq_ftrace_dump(int key, struct tty_struct *tty)
 }
 static struct sysrq_key_op sysrq_ftrace_dump_op = {
 	.handler	= sysrq_ftrace_dump,
-	.help_msg	= "dumpZ-ftrace-buffer",
+	.help_msg	= "dump-ftrace-buffer(Z)",
 	.action_msg	= "Dump ftrace buffer",
 	.enable_mask	= SYSRQ_ENABLE_DUMP,
 };