Merge pull request #502 from ethereum/hwwhww/per_block

Update `Beacon chain state transition function` chapter intro

specs/core/0_beacon-chain.md

@@ -779,12 +779,13 @@ def lmd_ghost(store: Store, start_state: BeaconState, start_block: BeaconBlock)
 
 ## Beacon chain state transition function
 
-We now define the state transition function. At a high level the state transition is made up of two parts:
+We now define the state transition function. At a high level the state transition is made up of three parts:
 
-1. The per-slot transitions, which happens every slot, and only affects a parts of the `state`.
-2. The per-epoch transitions, which happens in the last slot of every epoch (i.e. `(state.slot + 1) % EPOCH_LENGTH == 0`), and affects the entire `state`.
+1. The per-slot transitions, which happens every slot.
+2. The per-block transitions, which happens at every block.
+3. The per-epoch transitions, which happens at every epoch boundary (i.e. `(state.slot + 1) % EPOCH_LENGTH == 0`).
 
-The per-slot transitions generally focus on verifying aggregate signatures and saving temporary records relating to the per-slot activity in the `BeaconState`. The per-epoch transitions focus on the [validator](#dfn-validator) registry, including adjusting balances and activating and exiting [validators](#dfn-validator), as well as processing crosslinks and managing block justification/finalization.
+The per-slot transitions focus on the slot counter and block roots records updates; the per-block transitions generally focus on verifying aggregate signatures and saving temporary records relating to the per-block activity in the `BeaconState`; the per-epoch transitions focus on the [validator](#dfn-validator) registry, including adjusting balances and activating and exiting [validators](#dfn-validator), as well as processing crosslinks and managing block justification/finalization.
 
 ### Helper functions