Doc update to explain the request presented via Java Issue #351 (#3005)

Author: nd4p90x

src/connections/spec/group.md

@@ -170,4 +170,7 @@ The following are the reserved traits we have standardized:
 
 **Note:** You might be used to some destinations recognizing special properties differently. For example, Mixpanel has a special `track_charges` method for accepting revenue. Luckily, you don't have to worry about those inconsistencies. Just pass us `revenue`.  **We'll handle all of the destination-specific conversions for you automatically.** Same goes for the rest of the reserved properties.
 
+If you pass these values, `on null` will throw a `NullPointerException`.
+You may continue to set values inside the trait.  If you do so, this would work the same as the rules do with NoSQL data. If you had set a value previously for a user and on the next request you sent the same value of that property as `on null`, it will be replaced by `null`, but if you do not send that property, the original value is persisted.
+
 **Traits are case-insensitive**, so in Javascript you can match the rest of your camel-case code by sending `createdAt`, and in Ruby you can match your snake-case code by sending `created_at`. That way the API never seems alien to your code base.