docs: Improve discussion of this_cpu_ptr(), add raw_cpu_ptr()

docs: Improve discussion of this_cpu_ptr(), add raw_cpu_ptr()

Most of the this_cpu_*() operations may be used in preemptible code,
but not this_cpu_ptr(), and for good reasons.  Therefore, better explain
the reasons and call out raw_cpu_ptr() as an alternative in certain very
special cases.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: <linux-doc@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>