Merge tag 'docs-4.16-fix' of git://git.lwn.net/linux

Pull Documentation build fix from Jonathan Corbet:
 "The Sphinx 1.7 release broke the build process for reasons that are
  mostly our fault.

  This is a single fix cherry-picked from docs-next that restores docs
  buildability for all supported Sphinx versions"

* tag 'docs-4.16-fix' of git://git.lwn.net/linux:
  Documentation/sphinx: Fix Directive import error