mm: document mmu_notifier_invalidate_range_start_nonblock()

Document what mmu_notifier_invalidate_range_start_nonblock() is for.  Also
add a __must_check annotation to signal that callers must bail out if a
notifier vetoes the operation.

Link: https://lkml.kernel.org/r/20230918201832.265108-1-jannh@google.com


Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>