add doc string for new slashing helper funtions

specs/core/0_beacon-chain.md

@@ -1043,6 +1043,11 @@ def verify_slashable_vote_data(state: BeaconState, vote_data: SlashableVoteData)
 ```python
 def is_double_vote(attestation_data_1: AttestationData,
                    attestation_data_2: AttestationData) -> bool
+    """
+    Assumes ``attestation_data_1`` is distinct from ``attestation_data_2``.
+    Returns True if the provided ``AttestationData`` are slashable
+    due to a 'double vote'.
+    """
     return attestation_data_1.slot == attestation_data_2.slot
 ```
 
@@ -1051,6 +1056,13 @@ def is_double_vote(attestation_data_1: AttestationData,
 ```python
 def is_surround_vote(attestation_data_1: AttestationData,
                      attestation_data_2: AttestationData) -> bool
+    """
+    Assumes ``attestation_data_1`` is distinct from ``attestation_data_2``.
+    Returns True if the provided ``AttestationData`` is slashable
+    due to a 'surround vote'.
+    Note: parameter order matters as this function only checks
+    that ``attestation_data_1`` surrounds ``attestation_data_2``.
+    """
     return (
         (attestation_data_1.justified_slot < attestation_data_2.justified_slot) and
         (attestat_data_2.justified_slot + 1 == attestation_data_2.slot) and