io_ordering.txt: standardize document format

Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:

- Add a title;
- mark literal-blocks as such.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/io_ordering.txt

+==============================================
+Ordering I/O writes to memory-mapped addresses
+==============================================
+
 On some platforms, so-called memory-mapped I/O is weakly ordered.  On such
 platforms, driver writers are responsible for ensuring that I/O writes to
 memory-mapped addresses on their device arrive in the order intended.  This is
@@ -8,39 +12,39 @@ critical section of code protected by spinlocks.  This would ensure that
 subsequent writes to I/O space arrived only after all prior writes (much like a
 memory barrier op, mb(), only with respect to I/O).
 
-A more concrete example from a hypothetical device driver:
+A more concrete example from a hypothetical device driver::
 
 		...
-CPU A:  spin_lock_irqsave(&dev_lock, flags)
-CPU A:  val = readl(my_status);
-CPU A:  ...
-CPU A:  writel(newval, ring_ptr);
-CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
+	CPU A:  spin_lock_irqsave(&dev_lock, flags)
+	CPU A:  val = readl(my_status);
+	CPU A:  ...
+	CPU A:  writel(newval, ring_ptr);
+	CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
 		...
-CPU B:  spin_lock_irqsave(&dev_lock, flags)
-CPU B:  val = readl(my_status);
-CPU B:  ...
-CPU B:  writel(newval2, ring_ptr);
-CPU B:  spin_unlock_irqrestore(&dev_lock, flags)
+	CPU B:  spin_lock_irqsave(&dev_lock, flags)
+	CPU B:  val = readl(my_status);
+	CPU B:  ...
+	CPU B:  writel(newval2, ring_ptr);
+	CPU B:  spin_unlock_irqrestore(&dev_lock, flags)
 		...
 
 In the case above, the device may receive newval2 before it receives newval,
-which could cause problems.  Fixing it is easy enough though:
+which could cause problems.  Fixing it is easy enough though::
 
 		...
-CPU A:  spin_lock_irqsave(&dev_lock, flags)
-CPU A:  val = readl(my_status);
-CPU A:  ...
-CPU A:  writel(newval, ring_ptr);
-CPU A:  (void)readl(safe_register); /* maybe a config register? */
-CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
+	CPU A:  spin_lock_irqsave(&dev_lock, flags)
+	CPU A:  val = readl(my_status);
+	CPU A:  ...
+	CPU A:  writel(newval, ring_ptr);
+	CPU A:  (void)readl(safe_register); /* maybe a config register? */
+	CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
 		...
-CPU B:  spin_lock_irqsave(&dev_lock, flags)
-CPU B:  val = readl(my_status);
-CPU B:  ...
-CPU B:  writel(newval2, ring_ptr);
-CPU B:  (void)readl(safe_register); /* maybe a config register? */
-CPU B:  spin_unlock_irqrestore(&dev_lock, flags)
+	CPU B:  spin_lock_irqsave(&dev_lock, flags)
+	CPU B:  val = readl(my_status);
+	CPU B:  ...
+	CPU B:  writel(newval2, ring_ptr);
+	CPU B:  (void)readl(safe_register); /* maybe a config register? */
+	CPU B:  spin_unlock_irqrestore(&dev_lock, flags)
 
 Here, the reads from safe_register will cause the I/O chipset to flush any
 pending writes before actually posting the read to the chipset, preventing