basics: add client-server build schema

Author: balat

tutos/dev/manual/basics.wiki

@@ -536,7 +536,18 @@ PPX annotations allow to split the code into these two programs:
 
 Same for {{{module%shared}}}, {{{open%shared}}}, {{{type%shared}}} etc.
 
+{{@@class="doc-img-col-width"@@files/tutorial/client-server.svg|Client-server build system}}
+
 ===Client values===
+Fragments of client code can be included in server (or shared) sections.
+
+Example:
+
+<<code language="ocaml" class="server"|
+  button ~a:[a_onclick [%client fun ev -> ... ]] [ ... ]
+ >>
+
+
 Client values can be declared within server or shared code as
    {{{[%client[ (<value> : <type>) ]}}}.
 The type annotation is almost always required.
@@ -735,7 +746,7 @@ Attributes like {{{a_onclick}}} in module {{{Eliom_content.Html.D}}} or {{{F}}}
 take a client side function as parameter:
 
 <<code language="ocaml" class="shared"|
-div ~a:[a_onclick [%client fun ev -> ... ]] [ ... ]
+button ~a:[a_onclick [%client fun ev -> ... ]] [ ... ]
 >>
 
 Module {{{Lwt_js_events}}} of Js_of_ocaml defines a way to bind browser events