[Attention] Clarify comment explaining attn_logits +1 dimension (#33427)

Signed-off-by: Francesco Fusco <ffu@zurich.ibm.com>

vllm/v1/attention/backends/mla/triton_mla.py

@@ -143,8 +143,8 @@ class TritonMLAImpl(MLACommonImpl[MLACommonMetadata]):
                 B,
                 q_num_heads,
                 num_kv_splits,
-                # NOTE(lucas) idk why the +1 is here but sglang has it so we
-                # just mirror that
+                # NOTE: the +1 stores the LogSumExp (LSE) that the stage2
+                # kernel uses to merge partial attention outputs across splits.
                 self.kv_lora_rank + 1,
             ),
             dtype=torch.float32,