sched/completions/Documentation: Add recommendation for dynamic and ONSTACK completions

To prevent dynamic completion objects from being de-allocated while still
in use, add a recommendation to embed them in long lived data structures.

Also add a note for the on-stack case that emphasizes the dangers of
the limited scope, and recommends dynamic allocation if scope limitations
are not clearly understood.

[ mingo: Minor touch-ups of the text, expanded it a bit to make the
         warnings Nicholas added more prominent. ]

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: john.garry@huawei.com
Link: http://lkml.kernel.org/r/1539697539-24055-1-git-send-email-hofrat@osadl.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>