Better document warning suppression code

tinyxml2.cpp

@@ -1769,7 +1769,8 @@ XMLDocument::XMLDocument( bool processEntities, Whitespace whitespace ) :
     _errorStr2( 0 ),
     _charBuffer( 0 )
 {
-    _document = this;	// avoid warning about 'this' in initializer list
+    // avoid VC++ C4355 warning about 'this' in initializer list (C4355 is off by default in VS2012+)
+    _document = this;
 }