Implemented #[doc(cfg(...))].

This attribute has two effects:

1. Items with this attribute and their children will have the "This is
   supported on **** only" message attached in the documentation.

2. The items' doc tests will be skipped if the configuration does not
   match.

src/librustdoc/html/render.rs

@@ -1950,6 +1950,14 @@ fn short_stability(item: &clean::Item, cx: &Context, show_reason: bool) -> Vec<S
         stability.push(format!("<div class='stab deprecated'>{}</div>", text))
     }
 
+    if let Some(ref cfg) = item.attrs.cfg {
+        stability.push(format!("<div class='stab portability'>{}</div>", if show_reason {
+            cfg.render_long_html()
+        } else {
+            cfg.render_short_html()
+        }));
+    }
+
     stability
 }
 

src/librustdoc/html/static/styles/main.css

@@ -152,6 +152,7 @@ a.test-arrow {
 
 .stab.unstable { background: #FFF5D6; border-color: #FFC600; }
 .stab.deprecated { background: #F3DFFF; border-color: #7F0087; }
+.stab.portability { background: #C4ECFF; border-color: #7BA5DB; }
 
 #help > div {
 	background: #e9e9e9;