[mangeshraut712]

Summary

Aligns the TMA multicast README with consumer Blackwell support.

Closes #966.

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[nihalpasham]

Thanks for looking into #966. Closing this PR because the consumer Blackwell README updates already landed on main:

• #1172 and #1242 already updated tma_multicast/README.md and src/main.rs with the RTX 5090 (sm_120a) transcript and the sm_90+ target gate.
• On sm_120a, ptxas does not emit hardware UTMALDG.2D.MULTICAST; it prints an advisory warning and lowers .multicast::cluster to the __cuda_syscall_cp_async_bulk_tensor_2d_tile_multicast software fallback, so claiming unconditional consumer Blackwell TMA multicast support is misleading.
• #966 remains open only to capture a real Hopper (sm_90a) hardware run.