[api] Make Node#internal_data private

Make the method protected instead of adding an scaring comment:

 _stay away from this_

This was probably made public, as it is needed in `def ==(other)`:

```
_data == other.internal_data
```

src/api/lib/activexml/node.rb

@@ -480,10 +480,10 @@ def find_matching(conds)
       return
     end
 
-    # stay away from this
-    def internal_data # nodoc
+    def internal_data
       _data
     end
+    protected :internal_data
 
     def marshal_dump
       raise "you don't want to put it in cache - never!"