Documenting sexp_register_c_type and sexp_make_cpointer.

doc/chibi.scrbl

@@ -904,6 +904,28 @@ to any inherited from the parent type \var{parent}.  If \var{parent} is false,
 inherits from the default \var{object} record type.
 }}
 
+\item{\ccode{sexp sexp_register_c_type(sexp ctx, sexp name, sexp finalizer)}
+\p{
+Shortcut to defines a new type as a wrapper around a C pointer.
+Returns the type object, which can be used with sexp_make_cpointer to
+wrap instances of the type.  The finalizer may be sexp_finalize_c_type
+in which case managed pointers are freed as if allocated with malloc,
+NULL in which case the pointers are never freed, or otherwise a
+procedure of one argument which should release any resources.
+}}
+
+\item{\ccode{sexp sexp_make_cpointer(sexp ctx, sexp_uint_t type_id, void* value, sexp parent, int freep)}
+\p{
+Creates a new instance of the type indicated by type_id wrapping
+value. If parent is provided, references to the child will also
+preserve the parent, important e.g. to preserve an enclosing struct
+when wrapped references to nested structs are still in use.  If freep
+is true, then when reclaimed by the GC the finalizer for this type,
+if any, will be called on the instance.
+
+You can retrieve the id from a type object with sexp_type_tag(type).
+}}
+
 ]
 
 See the C FFI for an easy way to automate adding bindings for C