Adding Tription JavaDoc

Author: bathalh

src/main/scala/spray/json/Tription.scala

@@ -1,6 +1,23 @@
 package spray.json
 
 /**
+  * A Triple-Option
+  *
+  * JavaScript (and JSON), unlike Java/Scala, allow undefined values, which are distinct from null values.
+  * For example, a PUT request may have a payload like this:
+  * <code>
+  *     { "id":"234565434567898789098765",
+  *       "field1": 7,
+  *       "field3: null,
+  *       "field4": undefined }
+  * </code>
+  * which would tell the server to update field1 to 7, set field3 to null, and leave field2 and field4 alone.
+  * With a standard scala `Option`, it is impossible to tell whether the payload of the request had field2, field3,
+  * and field4 null or undefined since any missing values translate to `None`.
+  *
+  * The Tription solves that problem by defining `Value` for present values, `Null` for null values, and
+  * `Undefined` for values which are missing or explicitly marked as undefined.
+  *
   * Created by bathalh on 2/19/16.
   */
 abstract class Tription[+T]