Improve the description of epoch end

specs/BlockchainLayer/EpochManager/Epoch.md

@@ -10,29 +10,42 @@ Within one epoch, validator assignment is based on block height: each height has
 each height and shard have a chunk producer.
 
 ### End of an epoch
-A block is defined to be the last block in its epoch if it's the genesis block or if the following conditions are met:
-- Let `estimated_next_epoch_start = first_block_in_epoch.height + epoch_length`
-- `block.height + 1 >= estimated_next_epoch_start`
+
+Let `estimated_next_epoch_start = first_block_in_epoch.height + epoch_length`
+
+A `block` is defined to be the last block in its epoch if it's the genesis block or if the following condition is met:
+
 - `block.last_finalized_height + 3 >= estimated_next_epoch_start`
 
 `epoch_length` is defined in `genesis_config` and has a value of `43200` height delta on mainnet (12 hours at 1 block per second).
 
+Since every final block must have two more blocks on top of it, it means that the last block in an epoch will have a height of at least `block.last_finalized_height + 2`, so for the last block it holds that `block.height + 1 >= estimated_next_epoch_start`. Its height will be at least `estimated_next_epoch_start - 1`.
+
+Note that an epoch only ends when there is a final block above a certain height. If there are no final blocks, the epoch will be stretched until the required final block appears. An epoch can potentially be longer than `epoch_length`.
+
+![Diagram of epoch end](epoch_end_diagram.png)
+
 ### EpochHeight
+
 Epochs on one chain can be identified by height, which is defined the following way:
+
 - Special epoch that contains genesis block only: undefined
 - Epoch starting from the block that's after genesis: `0` (NOTE: in current implementation it's `1` due to a bug, so there are two epochs with height `1`)
 - Following epochs: height of the parent epoch plus one
 
 ### Epoch id
+
 Every block stores the id of its epoch - `epoch_id`.
 
 Epoch id is defined as
+
 - For special genesis block epoch it's `0`
 - For epoch with height `0` it's `0` (NOTE: the first two epochs use the same epoch id)
 - For epoch with height `1` it's the hash of genesis block 
 - For epoch with height `T+2` it's the hash of the last block in epoch `T`
 
-### End of an epoch
+### Epoch end
+
 - After processing the last block of epoch `T`, `EpochManager` aggregates information from block of the epoch, and computes
 validator set for epoch `T+2`. This process is described in [EpochManager](EpochManager.md).
 - After that, the validator set rotates to epoch `T+1`, and the next block is produced by the validator from the new set