Documentation: this_cpu_ops.txt: Update description of this_cpu_ops

Update the description for per cpu operations to clarify use cases of
this_cpu operations and add considerations for remote access.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>