Preserve RDoc labeled lists in generated markdown

[skatkov]

Summary

RDoc labeled lists are currently flattened into plain paragraphs instead of being preserved as labeled/definition lists in generated markdown.

Reference

RDoc Markup reference: https://www.mikeperham.com/wp-content/uploads/2010/12/rdoc.html

Example source

# Label list probe
#
# [cat] small domestic animal
# [dog]
#   loyal friend
#
# alpha:: first item
# beta::
#   second item
class LabelListProbe
  # noop
  def call; end
end

Current markdown output

Label list probe

cat

small domestic animal

dog

loyal friend

alpha

first item

beta

second item

Expected behavior

The generated markdown should preserve the RDoc list structure instead of flattening labels and bodies into separate paragraphs.

For example, output along these lines would be much easier to consume:

cat:
small domestic animal

dog:
loyal friend

alpha:
first item

beta:
second item

or another markdown representation that keeps the label/body relationship intact.

[skatkov]

RDoc master comparison note: this appears fixed upstream if HTML definition-list output is acceptable for generated Markdown.

Tested with ruby/rdoc master at bc0baee787609f2205e8f1103f3e1d36b923184a. The same labeled-list probe now preserves structure as <dl class="rdoc-list label-list"> / <dl class="rdoc-list note-list"> instead of flattening labels and bodies into unrelated paragraphs.

Related upstream reference: https://github.com/ruby/rdoc/blob/master/doc/markup_reference/markdown.md#definition-lists