Merge pull request ipld#192 from filecoin-project/feat/post-and-seal-proofs-to-datastructures

add PoStProof description and note size of arrays

Author: whyrusleeping

data-structures.md

@@ -206,7 +206,11 @@ BlockHeader is a serialized `Block`.
 
 #### `SealProof`
 
-SealProof is an array of bytes.
+SealProof is a 384-element array of bytes.
+
+#### `PoStProof`
+
+PoStProof is a 192-element array of bytes.
 
 #### `TokenAmount`