rustdoc: whitelist the headers that get a § on hover.

Previously the :hover rules were making the links to the traits/types in
something like

    impl<K: Hash + Eq, V> ... { ... }

be displayed with a trailing `§` when hovered over. This commit
restricts that behaviour to specific headers, i.e. those that are known
to be section headers (like those rendered in markdown doc-comments, and
the "Modules", "Functions" etc. headings).

src/librustdoc/html/render.rs

@@ -1064,7 +1064,9 @@ fn item_module(w: &mut Writer, cx: &Context,
                 clean::ForeignStaticItem(..)   => ("ffi-statics", "Foreign Statics"),
                 clean::MacroItem(..)           => ("macros", "Macros"),
             };
-            try!(write!(w, "<h2 id='{id}'><a href=\"\\#{id}\">{name}</a></h2>\n<table>",
+            try!(write!(w,
+                        "<h2 id='{id}' class='section-link'>\
+                        <a href=\"\\#{id}\">{name}</a></h2>\n<table>",
                         id = short, name = name));
         }