kmod: add up-to-date explanations on the purpose of each asynchronous levels

There seem to be quite some confusions on the comments, likely due to
changes that came after them.

Now since it's very non obvious why we have 3 levels of asynchronous code
to implement usermodehelpers, it's important to comment in detail the
reason of this layout.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>