Docs/mm/damon: document exclusivity of special-purpose modules

Add a section in design.rst to explain that DAMON special-purpose kernel
modules (LRU_SORT, RECLAIM, STAT) run in an exclusive manner and return
-EBUSY if another is already running.

Update lru_sort.rst, reclaim.rst and stat.rst by adding cross-references
to this exclusivity rule at the end of their respective Example sections.

This change is motivated from another discussion [1].

Link: https://lkml.kernel.org/r/20260315162945.80994-1-sj@kernel.org
Link: https://lore.kernel.org/damon/20260314002119.79742-1-sj@kernel.org/T/#t [1]
Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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