Documentation: add description for FTRACE probe status

Now kprobe status have four flags, so kprobes.txt
should reflect all of them.

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>