Add note about POD compatibility of function-nested structs.

struct.dd

@@ -3,7 +3,9 @@ Ddoc
 $(SPEC_S Structs & Unions,
 
         $(P Whereas classes are reference types, structs are value types.
-        Any C struct can be exactly represented as a D struct.
+        Any C struct can be exactly represented as a D struct, except non-static
+        $(LINK2 #nested, function-nested D structs) which access the context of
+        their enclosing scope.
         In C++ parlance, a D struct is a
         $(DPLLINK glossary.html#pod, POD (Plain Old Data)) type,
         with a trivial constructor and destructor.