fix javaDoc (#1122)

gson/src/main/java/com/google/gson/FieldAttributes.java

@@ -136,9 +136,13 @@ public final class FieldAttributes {
   }
 
   /**
-   * This is exposed internally only for the removing synthetic fields from the JSON output.
+   * Returns the value of the field represented by this {@code Field}, on
+   * the specified object. The value is automatically wrapped in an
+   * object if it has a primitive type.
    *
-   * @return true if the field is synthetic; otherwise false
+   * @return the value of the represented field in object
+   * {@code obj}; primitive values are wrapped in an appropriate
+   * object before being returned
    * @throws IllegalAccessException
    * @throws IllegalArgumentException
    */