DevTools: [Documentation] Fix bug with rendering of returnValueDescription

Return value description happens to contain some markup in text, so it
should be represented with ArticleElement instead of string.

BUG=391593

Review URL: https://codereview.chromium.org/581353002

git-svn-id: svn://svn.chromium.org/blink/trunk@182336 bbb929c8-8fbe-4397-9dbb-9b2b20218538