update arch doc

docs/architecture/adr-020-block-size.md

@@ -5,7 +5,8 @@
 13-08-2018: Initial Draft
 15-08-2018: Second version after Dev's comments
 28-08-2018: Third version after Ethan's comments
-30-08-2018: AminoOverheadForBlock => MaxAminoOverheadForBlock, added MaxAminoOverheadForTx
+30-08-2018: AminoOverheadForBlock => MaxAminoOverheadForBlock
+31-08-2018: Bounding evidence and chain ID
 
 ## Context
 
@@ -44,12 +45,16 @@ are constants defined inside the `types` package:
 - MaxEvidenceBytes - 364 bytes
 - MaxHeaderBytes - 476 bytes (~276 bytes hashes + 200 bytes - 50 UTF-8 encoded
   symbols of chain ID 4 bytes each in the worst case + amino overhead)
-- MaxAminoOverheadForBlock - 4 bytes (assuming MaxHeaderBytes includes amino
+- MaxAminoOverheadForBlock - 8 bytes (assuming MaxHeaderBytes includes amino
   overhead for encoding header, MaxVoteBytes - for encoding vote, etc.)
 
+ChainID needs to bound to 50 symbols max.
+
+When reaping evidence, we use MaxBytes to calculate the upper bound (e.g. 1/10)
+to save some space for transactions.
+
 NOTE while reaping the `max int` bytes in mempool, we should account that every
 transaction will take `len(tx)+aminoOverhead`, where aminoOverhead=1-4 bytes.
-Therefore, MaxAminoOverheadForTx should be added.
 
 We should write a test that fails if the underlying structs got changed, but
 MaxXXX stayed the same.