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.
diff --git a/tinyxml2.h b/tinyxml2.h
index d0620cc..70a9851 100755
--- a/tinyxml2.h
+++ b/tinyxml2.h
@@ -122,9 +122,9 @@
 /* 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 @@
 

     /** 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