mm/damon: add a kernel-doc comment for damon_ctx->rnd_state

mm/damon: add a kernel-doc comment for damon_ctx->rnd_state

Fix below kernel document build warning:

    WARNING: ../include/linux/damon.h:909 struct member 'rnd_state' not described in 'damon_ctx'

Link: https://lore.kernel.org/20260628220808.98931-3-sj@kernel.org
Fixes: 9012c4e647df ("mm/damon: replace damon_rand() with a per-ctx lockless PRNG")
Signed-off-by: SJ Park <sj@kernel.org>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/4df95955-b255-4e5a-90c4-35db02f3111f@infradead.org
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>