fix Issue 3906 - Undefined struct and union declarations are not docu…

…mented

struct.dd

@@ -404,6 +404,29 @@ ABC x;
 )
 
 
+<h3>Opaque Structs and Unions</h3>
+
+	$(P Opaque struct and union declarations do not have a $(GLINK StructBody):)
+
+---
+struct S;
+union U;
+---
+
+	$(P The members are completely hidden to the user, and so the only operations
+	on those types are ones that do not require any knowledge of the contents
+	of those types. For example:)
+
+---
+struct S;
+S.sizeof; // error, size is not known
+S s;      // error, cannot initialize unknown contents
+S* p;     // ok, knowledge of members is not necessary
+---
+
+	$(P They can be used to implement the
+	$(LINK2 http://en.wikipedia.org/wiki/Opaque_pointer, PIMPL idiom).)
+
 
 <h3>Static Initialization of Structs</h3>