tracing: update documentation of snapshot utility

Now, "snapshot" file returns success on a reset of snapshot buffer
even if the buffer wasn't allocated, instead of returning EINVAL.
This patch updates snapshot desctiption according to the change.

Link: http://lkml.kernel.org/r/51399409.4090207@hitachi.com

Signed-off-by: Hiraku Toyooka <hiraku.toyooka.gu@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Documentation/trace/ftrace.txt

@@ -1873,7 +1873,7 @@ feature:
 
 	status\input  |     0      |     1      |    else    |
 	--------------+------------+------------+------------+
-	not allocated |(do nothing)| alloc+swap |   EINVAL   |
+	not allocated |(do nothing)| alloc+swap |(do nothing)|
 	--------------+------------+------------+------------+
 	allocated     |    free    |    swap    |   clear    |
 	--------------+------------+------------+------------+