rdoc.css: make label-list compact

"Labeled Lists" section in lib/rdoc/markup.rb states labeled-list
will be formatted in same lines.

```
Notice that blank lines right after the label are ignored in labeled lists:

  [one]

      definition 1

  [two]

      definition 2

produces the same output as

  [one]  definition 1
  [two]  definition 2
```

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66990 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rdoc/generator/template/darkfish/css/rdoc.css

@@ -261,6 +261,11 @@ ul.link-list .type {
   -webkit-border-radius: 5px;
 }
 
+dl.label-list dt {
+  float: left;
+  margin-right: 1em;
+}
+
 .calls-super {
   background: url(images/arrow_up.png) no-repeat right center;
 }