docs/mm: remove references to hmm_mirror ops and clean typos

Clean typos and remove the reference to the sync_cpu_device_pagetables()
callback since all hmm_mirror ops have been removed.

Fixes: a22dd506 ("mm/hmm: remove hmm_mirror and related")
Signed-off-by: Marco Pagani <marpagan@redhat.com>
Reviewed-by: Mika Penttilä <mpenttil@redhat.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230825133546.249683-1-marpagan@redhat.com