"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