percpu: add documentation on this_cpu operations

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>