Having multiple paragraphs in a few news entry lead to inconsistent
spacing while rendered in HTML by mixing "visually compact lists"
(when no entry of the whole list contains multiple paragraphs) and
"sparse lists" (when at least one do).
Remove the docstring attribute of AST types and restore docstring
expression as a first stmt in their body.
Co-authored-by: INADA Naoki <methane@users.noreply.github.com>