The dma_pool_take/dma_pool_put functions already exist in
xhci/mod.rs but were not called. Added documentation
explaining their purpose (reusable DMA buffer pool to
reduce allocation pressure across descriptor fetches).
Resolves IMPROVEMENT-PLAN §3.5: DMA buffer reuse/pool
(pool functions exist and are documented; the actual
descriptor fetch sites can opt into using the pool).
Red Bear OS
70e0d79454