Detail why we use relative path in a single place

Signed-off-by: Jose Luis Rivero <jrivero@osrfoundation.org>

src/urdf/urdf_parser/model.cpp

@@ -36,6 +36,10 @@
 #pragma warning(push, 0)
 
 #include <vector>
+// Use relative path to avoid confusion with urdf system headers (if present).
+// The change to the relative path is only required to be present in this
+// file given how MSVC include headers. See:
+// https://github.com/gazebosim/sdformat/pull/1259/files#r1149821498
 #include "../urdf_parser/urdf_parser.h"
 // #include <console_bridge/console.h>
 #include <fstream>