docs: changes varying number of messages to with null messages

It makes more sense here to say null messages rather than varying number of messages which would indicate that the the count of messages being encoded in the signature changes.

README.md

@@ -28,7 +28,7 @@ the [WASM wrapper](https://github.com/docknetwork/crypto-wasm).
       - [Terminology](#terminology)
       - [Examples](#examples)
         - [Selective disclosure](#selective-disclosure)
-        - [BBS signature over varying number of messages](#bbs-signature-over-varying-number-of-messages)
+        - [BBS signature over null-valued messages](#bbs-signatures-over-null-valued-messages)
         - [Multiple BBS signatures](#multiple-bbs-signatures)
         - [BBS signature together with accumulator membership](#bbs-signature-together-with-accumulator-membership)
         - [Getting a blind signature](#getting-a-blind-signature)
@@ -548,14 +548,18 @@ const verifierProofSpec = new ProofSpec(statements, ms, [], context);
 console.assert(proof.verify(verifierProofSpec, nonce).verified);
 ```
 
-##### BBS signatures over varying number of messages 
+##### BBS signatures over null-valued messages 
 
-The examples shown here have assumed that the number of messages for given signature params is fixed but that might not be always true. 
-An example is where some of the messages in the signature are null (like N/A) in certain signatures. Eg, when the messages are attributes
-in a credential that specifies the educational qualifications and institutes of a person, someone with a high school level education will 
-have N/A for attributes like university name, major, etc. One way to deal with it is to decide some sentinel value like 0 for all the N/A
-attributes and disclose those attributes while creating a proof. Other is to have certain attribute in the credential specify which attribute 
-indices that are N/A and always reveal this attribute. A complete example of the latter is shown in this [test](tests/composite-proofs/variable-number-of-messages.spec.ts).
+The examples above assumed all messages have values in them. However, in some cases, one or more attributes will be null within the credential.  
+An example in which some of the messages correspond to attributes with null values (e.g. N/A) is a education qualification credential of a person. Someone with a highschool-level education will 
+have N/A for attributes like university name, major, etc.
+
+One way to deal with this is to decide on some sentinel value like 0 or 'N/A' for all the null attributes
+and disclose those values to the verifier.  
+Another is to have a certain attribute (e.g. first message) in the credential specify which attribute 
+indices are null and always reveal this attribute.  
+A complete example of the latter is shown in this
+[test](tests/composite-proofs/variable-number-of-messages.spec.ts).
 
 ##### Multiple BBS signatures