Added some clarification and elaboration around sendMessage.

docs/getting-started-tutorial/6-adding-usernames.adoc

@@ -121,7 +121,10 @@ there's a possibility of a value not being set.]:
   def sendMessage = {
     var message: Box[ChatMessage] = Empty
 
-    "#new-message" #> SHtml.text(message, { body: String => message = Full(ChatMessage(username.is, body)) }) &
+    "#new-message" #> SHtml.text(
+      message,
+      { messageBody: String => message = Full(ChatMessage(username.get, messageBody)) } // <1>
+    ) &
     "type=submit" #> SHtml.submitButton(() => {
       for (body <- message) {
         messageEntries ::= message
@@ -130,10 +133,30 @@ there's a possibility of a value not being set.]:
   }
 ...
 ```
-
-We use a `for` comprehension to unpack the value of `message`. The body of that
-comprehension won't run unless `message` is a `Full` box containing a
-`ChatMessage` sent by the client.
+<1> Notably, `.get` is how you fetch the current value of a `SessionVar`. Don't
+    confuse it with the `.get` on `Option`, which is very dangerous! If you
+    prefer to use a method that is less easily confused with `Option`'s `.get`
+    (as many Lift developers and committers do), you can use `.is` instead, which
+    does the same thing.
+
+Let's break this one down to its composite parts. First, we update the handler
+for the `#new-message` text field. Before, the handler function was `message = _`;
+when `message` was a `String`, we could simply assign the message the user sent
+directly to it, and we were good to go. However, `message` is now a `ChatMessage`---
+it has to carry not only the message body that the user typed, but also their
+username. To do that, we write a complete handler function that takes in the
+body that the user submitted with the form and creates a  `ChatMessage` with
+that body and the current user's username.
+
+Note also that we wrap the newly-created `ChatMessage` in a `Full`. Before, we
+used an empty `String` as our starting value for the message. However, we can't
+do that anymore here. Our only option would be to use `null`, but `null` is very
+dangerous and, as a rule, we avoid using it in Scala. Instead, we use an `Empty`
+`Box`, and, when we receive a message body, we create a `Full` `Box` with the
+newly posted `ChatMessage`. As a result of the wrapping `Box`, we have to update
+the submission handler. We use a `for` comprehension to unpack the value of
+`message`. The body of that comprehension won't run unless `message` is `Full`,
+so we can't try to insert an empty message into the message list.
 
 Now that we have a reasonably nice chat system with actual usernames, it's time
 to look at the underlying issue of *consistency*. If two users posted a chat