sched/wait: Clean up wait.h details a bit

Since we are changing wait.h profoundly, use the opportunity to:

 - add a sentence to explain what this file is about
 - remove whitespace noise
 - prettify weird looking line break fixup attempts
 - standardize type definition and initialization sequences
 - use consistent style details

No code is changed.
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/n/tip-O8dIie5swnctqpupakatvqyq@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>