code comments to clarify when/why base_path is needed at parse time when loading from a string

src/libxml2_loader.cpp

@@ -107,7 +107,7 @@ public:
                                    base_path + "': file or directory does not exist");
             }
         }
-
+        // NOTE: base_path here helps libxml2 resolve entities correctly: https://github.com/mapnik/mapnik/issues/440
         xmlDocPtr doc = xmlCtxtReadMemory(ctx_, buffer.data(), buffer.length(), base_path.c_str(), encoding_, options_);
 
         load(doc, node);

src/rapidxml_loader.cpp

@@ -97,8 +97,9 @@ public:
         }
     }
 
-    void load_string(std::string const& buffer, xml_node &node, std::string const & base_path )
+    void load_string(std::string const& buffer, xml_node &node, std::string const & )
     {
+        // Note: base_path ignored because its not relevant - only needed for xml2 to load entities (see libxml2_loader.cpp)
         load_array(std::string(buffer), node);
     }
 private: