Clarify meaning of 'empty'

When `XMLDocument::Value()` is called, we intend to return NULL (`( const char* )0`). State that explicitly in the documentation, so as to disambiguate between "empty string" (`""`) and NULL.

tinyxml2.h

@@ -708,7 +708,7 @@ public:
 
     /** The meaning of 'value' changes for the specific type.
     	@verbatim
-    	Document:	empty
+    	Document:	empty (NULL is returned, not an empty string)
     	Element:	name of the element
     	Comment:	the comment text
     	Unknown:	the tag contents