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

@@ -122,9 +122,9 @@ inline int TIXML_SNPRINTF( char* buffer, size_t size, const char* format, ... )
 /* Versioning, past 1.0.14:
 	http://semver.org/
 */
-static const int TIXML2_MAJOR_VERSION = 3;
-static const int TIXML2_MINOR_VERSION = 0;
-static const int TIXML2_PATCH_VERSION = 0;
+static const int TIXML2_MAJOR_VERSION = 3;
+static const int TIXML2_MINOR_VERSION = 0;
+static const int TIXML2_PATCH_VERSION = 0;
 
 namespace tinyxml2
 {
@@ -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