[Misc]add some explanations for BlockHashType (vllm-project#11847)

Signed-off-by: Bowen Wang <abmfy@icloud.com>

vllm/v1/core/kv_cache_utils.py

@@ -11,8 +11,10 @@
 
 class BlockHashType(NamedTuple):
     """Hash value of a block (int), the token IDs in the block, and extra keys.
-    The reason we keep a tuple of token IDs and extra keys is to make sure
-    no hash collision happens when the hash value is the same.
+    We keep a tuple of token IDs and extra keys to reduce the likelihood of
+    hash collisions when the hash value is the same. But please note that 
+    hash collisions can still theoretically occur, albeit with an extremely 
+    low probability.
     """
     # Hash value of the block in an integer.
     hash_value: int