mm: mmu_notifier: fix and extend kerneldoc

mm: mmu_notifier: fix and extend kerneldoc

Fix W=1 compile warnings (invalid kerneldoc):

    mm/mmu_notifier.c:187: warning: Function parameter or member 'interval_sub' not described in 'mmu_interval_read_bgin'
    mm/mmu_notifier.c:708: warning: Function parameter or member 'subscription' not described in 'mmu_notifier_registr'
    mm/mmu_notifier.c:708: warning: Excess function parameter 'mn' description in 'mmu_notifier_register'
    mm/mmu_notifier.c:880: warning: Function parameter or member 'subscription' not described in 'mmu_notifier_put'
    mm/mmu_notifier.c:880: warning: Excess function parameter 'mn' description in 'mmu_notifier_put'
    mm/mmu_notifier.c:982: warning: Function parameter or member 'ops' not described in 'mmu_interval_notifier_insert'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: http://lkml.kernel.org/r/20200728171109.28687-4-krzk@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>