sched/rt: Add comments describing the RT IPI pull method

sched/rt: Add comments describing the RT IPI pull method

While looking into optimizations for the RT scheduler IPI logic, I realized
that the comments are lacking to describe it efficiently. It deserves a
lengthy description describing its design.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Clark Williams <williams@redhat.com>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170228155030.30c69068@gandalf.local.home
[ Small typographical edits. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>