Change the document about iowait

The iowait is not reliable by reading from /proc/stat, so this
method to get iowait is not suggested. And we mark it in the
document.
Signed-off-by: NCao Jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: NChao Fan <fanc.fnst@cn.fujitsu.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/filesystems/proc.txt

@@ -1305,7 +1305,16 @@ second).  The meanings of the columns are as follows, from left to right:
 - nice: niced processes executing in user mode
 - system: processes executing in kernel mode
 - idle: twiddling thumbs
-- iowait: waiting for I/O to complete
+- iowait: In a word, iowait stands for waiting for I/O to complete. But there
+  are several problems:
+  1. Cpu will not wait for I/O to complete, iowait is the time that a task is
+     waiting for I/O to complete. When cpu goes into idle state for
+     outstanding task io, another task will be scheduled on this CPU.
+  2. In a multi-core CPU, the task waiting for I/O to complete is not running
+     on any CPU, so the iowait of each CPU is difficult to calculate.
+  3. The value of iowait field in /proc/stat will decrease in certain
+     conditions.
+  So, the iowait is not reliable by reading from /proc/stat.
 - irq: servicing interrupts
 - softirq: servicing softirqs
 - steal: involuntary wait