percpu: add documentation on this_cpu operations
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 <ef3ecccf258fa062c5c6521a4887d40541963af7@linux.com>
Signed-off-by: Tejun Heo <546b05909706652891a87f7bfe385ae147f61f91@kernel.org>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas