Fixed mistakes in layouts/rendering guide about yield

yield(:unspecified_block) actually returns true even if :unspecified_block never
exists. This means you can't use the form yield(:unspecified_block) or yield.

railties/guides/source/layouts_and_rendering.textile

@@ -1179,14 +1179,14 @@ On pages generated by +NewsController+, you want to hide the top menu and add a
 <% end %>
 <% content_for :content do %>
   <div id="right_menu">Right menu items here</div>
-  <%= yield(:news_content) or yield %>
+  <%= content_for?(:news_content) ? yield(:news_content) : yield %>
 <% end %>
 <%= render :template => 'layouts/application' %>
 </erb>
 
 That's it. The News views will use the new layout, hiding the top menu and adding a new right menu inside the "content" div.
 
-There are several ways of getting similar results with different sub-templating schemes using this technique. Note that there is no limit in nesting levels. One can use the +ActionView::render+ method via +render :template => 'layouts/news'+ to base a new layout on the News layout. If you are sure you will not subtemplate the +News+ layout, you can replace the +yield(:news_content) or yield+ with simply +yield+.
+There are several ways of getting similar results with different sub-templating schemes using this technique. Note that there is no limit in nesting levels. One can use the +ActionView::render+ method via +render :template => 'layouts/news'+ to base a new layout on the News layout. If you are sure you will not subtemplate the +News+ layout, you can replace the +content_for?(:news_content) ? yield(:news_content) : yield+ with simply +yield+.
 
 h3. Changelog