DevTools: [Documentation] Add signature section for renderer

Drive-by fixes:
1) Fix constants documentation detection
2) Fix inherited Function methods problem

BUG=391593

Review URL: https://codereview.chromium.org/525363003

git-svn-id: svn://svn.chromium.org/blink/trunk@181648 bbb929c8-8fbe-4397-9dbb-9b2b20218538