Add docstring for hash_eth2

Co-Authored-By: ChihChengLiang <[email protected]>

Author: hwwhww

eth/beacon/utils/hash.py

@@ -3,4 +3,9 @@
 
 
 def hash_eth2(data: bytes) -> Hash32:
+    """
+    Return Keccak-256 hashed result.
+    Note: it's a placeholder and we aim to migrate to a S[T/N]ARK-friendly hash function in
+    a future Ethereum 2.0 deployment phase.
+    """
     return Hash32(keccak(data))