polish testing non-public method

docs/how-to-test-a-contract.md

@@ -73,9 +73,10 @@ const deployTx = await instance.deploy(1)
 console.log('Demo contract deployed: ', deployTx.id)
 ```
 
-## Call a Public Method
+## Call a Method
 
-You can call a contract's public `@method` on the blockchain as follows:
+### Public
+You can call a contract's public `@method` as [before](./how-to-deploy-and-call-a-contract/how-to-deploy-and-call-a-contract.md#contract-call):
 
 ```ts
 // build and send tx by calling `unlock()` on `methods` object.
@@ -84,9 +85,9 @@ await instance.methods.unlock(
 )
 ```
 
-## Call a Non-Public Method
+### Non-Public
 
-It is also possible to call non-public methods locally.
+You can also call non-public methods.
 
 Let's add a non-public method to our contract:
 
@@ -97,20 +98,29 @@ hashMessage(message: ByteString): ByteString {
 }
 ```
 
-You can now call this method locally like the following:
+You can now call this method like the following:
 
 ```ts
 const message: ByteString = toByteString('hello world')
 const hashRes: ByteString = instance.hashMessage(message)
 ```
 
+Note the absence of `.methods` after `instance`, compared to a public method.
+
 If the method is static, it can be called like this:
 
 ```ts
-const hashRes: ByteString = Demo.hashMessage(message)
+@method()
+static hashMessageStatic(message: ByteString): ByteString {
+    return sha256(message)
+}
+```
+
+```ts
+const hashRes: ByteString = Demo.hashMessageStatic(message)
 ```
 
-It should be noted that non-public methods are only directly callable off-chain, i.e. for testing. On-chain, they can only be invoked through a public method.
+It should be noted that non-public methods are only directly callable off-chain, e.g., for testing. On chain, they can only be invoked through a public method.
 
 ## Integrate with a testing framework