sched/swait: Document it clearly that the swait facilities are special and shouldn't be used

We currently welcome using swait over wait whenever possible because
it is a slimmer data structure. However, Linus has made it very clear
that he does not want this used, unless under very specific RT scenarios
(such as current users).

Update the comments before kernel hipsters start thinking swait is the
cool thing to do.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dave@stgolabs.net
Cc: wagi@monom.org
Link: http://lkml.kernel.org/r/20171020171346.24445-1-dave@stgolabs.net


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