doc: completion: context, scope and language fixes

Fix for imprecise/wrong statements on context in which wait_for_completion*()
can be called, updated notes on "going out of scope" problems and some
language fixups.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>