Merge branch 'refs/heads/t/doc-improvements' into next

* refs/heads/t/doc-improvements:
  Switch from grutatxt to markdown.
  INSTALL: Link to the gengetopt web page.
  manual: Fix a whitespace issue.
  manual: Add example for interactive mode.
  manual: Add two more examples.
  manual: Add example query for omitting a directory.
  manual: Add short option example.
  manual: Remove pointless "time" prefix.
  manual: Improve documentation of --output.