fix Issue 23699 - ImportC: Unclear documentation that struct/union/enum introduce implicit typedefs (#3581)

Author: WalterBright

spec/importc.dd

@@ -420,6 +420,36 @@ int myprintf(char *, ...) asm("printf");
     $(P Any declarations in scope can be accessed, not just
     declarations that lexically precede a reference.)
 
+    $(H3 $(LNAME2 cpp-tag-symbols, C++ Style Tag Symbols))
+
+    $(P In C++, `struct`, `union` or `enum` tag symbols can be accessed without needing
+    to be prefixed with the `struct`, `union` or `enum` keywords, as long
+    as there is no other declaration with the same name at the same scope.
+    ImportC behaves the same way.)
+
+    $(P For example, the following code is accepted by both C++ and ImportC:)
+
+$(CCODE
+struct s { int a; };
+
+void g(int s)
+{
+    struct s* p = (struct s*)malloc(sizeof(struct s));
+    p->a = s;
+}
+)
+
+    $(P Whereas this is rejected by both C++ and ImportC, for the same reason.)
+
+$(CCODE
+struct s { int a; };
+
+void g(int s)
+{
+    s* p = (s*)malloc(sizeof(s));
+    p->a = s;
+}
+)
 
     $(H3 $(LNAME2 ctfe, Compile Time Function Execution))