fix(content): fix BLS signature aggregation language

1. Messages, not transactions. I only fixed this for BLS signatures, we need to
   fix the rest of the spec separately.
2. We're relying on the first condition, not the second.
3. We're not "aggregating the transaction's signature of a block". Transactions
   don't sign blocks. We're aggregating all BLS message signatures within a
   single block.
4. I removed the final sentence about transaction validity. It doesn't seem to
   say anything useful, and I'm not entirely sure what it was trying to convey.

content/algorithms/crypto/signatures.md

@@ -114,7 +114,7 @@ candidates in recent blockchain systems:
   terms of grinding, which is a real concern in recent proof of stake systems.
 - BLS signatures are aggregatable: one can aggregate signatures from different
   signers into one single signature. This feature enables drastically saving
-  space on the blockchain, especially when aggregating user transactions.
+  space on the blockchain, especially when aggregating user messages.
 
 **Aggregation Functionality**: The aggregation functionality is commutative and
 associative, enabling _partial_ aggregation. For example, given
@@ -130,14 +130,12 @@ of measures, as explained [here](https://crypto.stanford.edu/~dabo/pubs/papers/B
 - Prove knowledge of the secret key
 - Use a modified scheme (such as [BLS Multi Sig](https://crypto.stanford.edu/~dabo/pubs/papers/BLSmultisig.html))
 
-Fortunately, Filecoin can enforce the second condition to safely use the
+Fortunately, Filecoin can enforce the first condition to safely use the
 aggregation property:
-Filecoin uses aggregation only for aggregating the transaction's signature of a
-block. Since Filecoin uses the account model to represent the state of the
-chain, each message for a given signer is used in combination with a nonce to
-avoid replay attacks. As a direct consequence, every transaction's message is
-unique thereby the aggregation is done on distinct messages. Obviously, the
+Filecoin uses aggregation only for aggregating message signatures within a
+single block. Since Filecoin uses the account model to represent the state of
+the chain, each message for a given signer is used in combination with a nonce
+to avoid replay attacks. As a direct consequence, every message is unique
+thereby the aggregation is done on distinct messages. Obviously, the
 **assumption** here is that the block producer **enforces that distinction** and
-the other miners will **check all transactions** to make sure they are valid.
-The validity of a transaction in Filecoin's context implies that the signature
-is correctly formed over the message with the correct nonce.
+the other miners will **check all messages** to make sure they are valid.