Documentation/userfaultfd: document RWP working set tracking

Add an admin-guide section covering UFFDIO_REGISTER_MODE_RWP:

  - sync and async fault models;
  - UFFDIO_RWPROTECT semantics;
  - UFFD_FEATURE_RWP_ASYNC;
  - UFFDIO_SET_MODE runtime mode flips.

It also covers typical VMM working-set-tracking workflow from detection
loop through sync-mode eviction and back to async.

Link: https://lore.kernel.org/20260708111417.173443-16-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Assisted-by: Claude:claude-opus-4-6
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: James Houghton <jthoughton@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: kernel test robot <lkp@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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