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>

Documentation/kprobes.txt

@@ -702,7 +702,8 @@ a virtual address that is no longer valid (module init sections, module
 virtual addresses that correspond to modules that've been unloaded),
 such probes are marked with [GONE]. If the probe is temporarily disabled,
 such probes are marked with [DISABLED]. If the probe is optimized, it is
-marked with [OPTIMIZED].
+marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
+[FTRACE].
 
 /sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.