Document the rationale and the way to use this_cpu operations.
V2: Improved after feedback from Randy Dunlap
v3: Further spelling fixes from Randy. Paragraphs refilled to 75
column.
tj: Added .txt file extension to the document.
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
