sched/core: Clean up comments

Refresh the comments in the core scheduler code:

 - Capitalize sentences consistently

 - Capitalize 'CPU' consistently

 - ... and other small details.

Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NIngo Molnar <mingo@kernel.org>