mm/damon/core: document damon_call()/damon_start() race hang issue

Let's suppose damon_start() and damon_call() are executed in parallel
for the same DAMON context.  Then, damon_call() could show
ctx->damon_calls_obsolete set while ctx->kdamond is unset.  If
damon_start() sets ctx->kdamond before damon_call() starts the
cancelling, damon_call() can indefinitely hang.  No DAMON API caller
does such parallel execution of damon_start() and damon_call(), so the
issue doesn't exist.  But who knows what will happen in future.  Add a
clarification comment for caution.

Link: https://lore.kernel.org/20260917142210.90829-6-sj@kernel.org
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Asier Gutierrez <gutierrez.asier@huawei-partners.com>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@davidgow.net>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>

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