add debug_print_doc command

[flyerhzm]

the debug_print_doc command returns the debug print doc, like

[
  group([
    group(["Config", "::", "Download", indent([softline, ".", "new"])]),
    group([
      "(",
      indent([
        softline,
        "'",
        "rubocop-config-prettier",
        "'",
        ",",
        line,
        group([group(["filename:", indent([line, "'", "rubocop.yml", "'"])])])
      ]),
      softline,
      ")"
    ]),
    indent([softline, ".", "perform"])
  ]),
  hardline,
  breakParent
];

it helps to check internal behaviors.

[kddnewton]

Merging this, I'm going to rename this just to doc though to be consistent.