Docs/mm/damon/design: document the power-of-two limitation for addr_unit

The min_region_sz is set as max(DAMON_MIN_REGION_SZ / addr_unit, 1). 
DAMON_MIN_REGION_SZ is the same to PAGE_SIZE, and addr_unit is what the
user can arbitrarily set.  Commit c80f46ac228b ("mm/damon/core: disallow
non-power of two min_region_sz") made min_region_sz to always be a power
of two.  Hence, addr_unit should be a power of two when it is smaller than
PAGE_SIZE.  While 'addr_unit' is a user-exposed parameter, the rule is not
documented.  This can confuse users.  Specifically, if the user sets
addr_unit as a value that is smaller than PAGE_SIZE and not a power of
two, the setup will explicitly fail.

Document the rule on the design document.  Usage documents reference the
design document for detail, so updating only the design document should
suffice.

Link: https://lkml.kernel.org/r/20260307194222.202075-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
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: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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