whisper: detail the internals of the JS example (#19481)

docs/_whisper/Whisper-js-example.md

@@ -4,7 +4,63 @@ title: Whisper JavaScript example
 
 [This link](https://github.com/gballet/whisper-chat-example) contains a full-fledged example of how to use Whisper in a small chat application.
 
-Let's now have a look at the `sendMessage` function:
+The app is a simple Vue single page application that works in several steps. In the first step, the user configures the RPC service that they wish to connect to. Typically, this would be a `geth` client with the `--shh` option enabled.
+
+Then one can install the application locally by typing:
+
+```
+$ git clone https://github.com/gballet/whisper-chat-example
+$ cd whisper-chat-example
+$ npm install
+```
+
+The application is then started by typing:
+
+```
+$ npm run dev
+```
+
+The application will then be available at `http://localhost:8080` (Note the http here, which is because it's a **demo application** and should not be used in production)
+
+## User workflow
+
+The app starts by asking the user for their user name and whether they want a symmetric or asymmetric connection. If it's asymmetric, the geth server will propose a public key. If it's symmetric, a key has to be provided.
+
+Then the user presses "Start" and the conversation begins.
+
+## The app's state
+
+```javascript
+                let data = {
+                        msgs: [],
+                        text: "",
+                        symKeyId: null,
+                        name: "",
+                        asymKeyId: null,
+                        sympw: "",
+                        asym: true,
+                        configured: false,
+                        topic: defaultTopic,
+                        recipientPubKey: defaultRecipientPubKey,
+                        asymPubKey: ""
+                };
+```
+
+This is how the current, transient, state of the application is represented:
+
+  * `msgs` is the list of messages in the current conversation
+  * `text` contains the text that the current user is typing
+  * `name` is the name of the current user, which is used to identify them in conversations
+  * `asymKeyId` and `symKeyId` represent handles to the corresponding keys in `geth`'s memory
+  * `recipientPubKey` is a hex string representing the public key that an asymmetric message is sent to
+  * `topic` is a hex string representing the message's topic
+  * `asymPubKey` is a hex string representing the user's own public key
+  * `configured` is a flag that is set to true when the user has choosen either a public key or a symmetric key, and a user name
+  * `sympw` contains the symmetric password
+
+## The `sendMessage` callback
+
+The `sendMessage` callback is called every time the user clicks on "Send" or presses the return key. It is responsible for creating the RPC request that instructs the `geth` node to encrypt and send the message.
 
 ```javascript
 sendMessage() {
@@ -17,6 +73,7 @@ sendMessage() {
     };
 
     // (code elided for clarity)
+    // ...
 
     // Create the data object that will be sent to the RPC endpoint.
     let postData = {
@@ -39,5 +96,14 @@ sendMessage() {
     this.shh.post(postData);
 
     // (code elided for clarity)
+    // ...
 }
-```
+```
+
+The `msg` object is created. The format chosen for the object is specific to this demo application. It just contains a text and the name of the sender. This is obviously not secure enough for a real-world application.
+
+That object is converted to a string and then encoded as a hexadecimal string, in the `payload` member of the request's `POST` data object. Other fields include the `topic` of the message, how much work the sending server should do and other parameters.
+
+Next, depending whether the "asymmetric" checkbox has been ticked, the value of `this.asym` will be true or false. Based on this, the system will update the request object with the relevant information.
+
+Finally, the request is being sent with `this.shh.post(postData)`, which calls Web3's `shh.post` function to send the message.