auto merge of #13776 : adrientetar/rust/rustdoc-fix, r=brson

- Closes #13591. Relevant example: http://adrientetar.legtux.org/cached/rust-docs/struct.CChars.htm
(Had to use `!important` to override CSS selector precedence, namely matching over parent class.)
- Implement changes from #13780 feedback, namely:
  * Changed font-size from 18px to 15px
  * Reintroduced gray background for code samples
  * Tightened up the margins
- Fix point 1 and point 4 of #13804.

Samples:

- [enum.FileType](http://adrientetar.legtux.org/cached/rust-docs/enum.FileType.htm)
- [struct.CChars](http://adrientetar.legtux.org/cached/rust-docs/struct.CChars.htm)
- [std](http://adrientetar.legtux.org/cached/rust-docs/std.htm)
- [std::io](http://adrientetar.legtux.org/cached/rust-docs/io.htm).

r? @brson

src/librustdoc/html/render.rs

@@ -1121,7 +1121,7 @@ fn item_module(w: &mut Writer, cx: &Context,
                 clean::MacroItem(..)           => ("macros", "Macros"),
             };
             try!(write!(w,
-                        "<h2 id='{id}' class='section-link'>\
+                        "<h2 id='{id}' class='section-header'>\
                         <a href=\"\\#{id}\">{name}</a></h2>\n<table>",
                         id = short, name = name));
         }