Add further documentation/info on the smartptr feature to the shared_ptr library file

[skip ci]

Lib/shared_ptr.i

@@ -4,6 +4,13 @@
 // to use a pointer to the smart pointer of the type, rather than the usual pointer to the underlying type.
 // So for some type T, shared_ptr<T> * is used rather than T *.
 
+// Another key part of the implementation is the smartptr feature:
+//   %feature("smartptr") T { shared_ptr<T> }
+// This feature marks the class T as having a smartptr to it (the shared_ptr<T> type). This is then used to
+// support smart pointers and inheritance. Say class D derives from base B, then shared_ptr<D> is marked
+// with a fake inheritance from shared_ptr<B> in the type system if the "smartptr" feature is used on both
+// B and D. This is to emulate the conversion of shared_ptr<D> to shared_ptr<B> in the target language.
+
 // shared_ptr namespaces could be boost or std or std::tr1
 // For example for std::tr1, use:
 // #define SWIG_SHARED_PTR_NAMESPACE std