add clarifying comment for genesis skip conditions

specs/phase0/beacon-chain.md

@@ -1305,6 +1305,8 @@ def get_attesting_balance(state: BeaconState, attestations: Sequence[PendingAtte
 
 ```python
 def process_justification_and_finalization(state: BeaconState) -> None:
+    # Initial FFG checkpoint values have a `0x00` stub for `root`.
+    # Skip updating FFG checkpoints in first two epochs to avoid corner cases that might result in modifying this stub.
     if get_current_epoch(state) <= GENESIS_EPOCH + 1:
         return
 
@@ -1512,6 +1514,7 @@ def get_attestation_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence
 
 ```python
 def process_rewards_and_penalties(state: BeaconState) -> None:
+    # No rewards are applied at end of `GENESIS_EPOCH` because rewards are for work done in the previous epoch
     if get_current_epoch(state) == GENESIS_EPOCH:
         return