Documentation/atomic_ops.txt: convert to ReST markup

... and move to core-api folder.
Signed-off-by: NSilvio Fricke <silvio.fricke@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -8,6 +8,7 @@ Kernel and driver related documentation.
    :maxdepth: 1
 
    assoc_array
+   atomic_ops
    local_ops
    workqueue
 

Documentation/process/volatile-considered-harmful.rst

+
+.. _volatile_considered_harmful:
+
 Why the "volatile" type class should not be used
 ------------------------------------------------