Explained GetRoot<Monster> vs GetMonster.

Change-Id: Ic302096c07551b5c1aff0b261a329c46964cf337

docs/source/Tutorial.md

@@ -1384,6 +1384,8 @@ won't work**
 
   // `monster` is of type `Monster *`.
   // Note: root object pointers are NOT the same as `buffer_pointer`.
+  // `GetMonster` is a convenience function that calls `GetRoot<Monster>`,
+  // the latter is also available for non-root types.
 ~~~
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