Decide: does RuleLike::check owe its caller source order?

`RuleLike::check` returns the violations a rule found in whatever order the rule
walked the document, and nothing says what that order is. MD027 can now return
them with a later line before an earlier one.

## Reproduction

```markdown
> > a
> b
>
> c
```

`MD027::check` returns `4:4` and then `2:4`. `doc.ast.descendants()` reaches the
outer quote first, so the outer quote's later block on line 4 is measured before
the walk reaches the nested quote, whose lazily continued line 2 belongs to the
inner one.

`mado check` is unaffected: `Command::check` sorts with
`config.lint.output_format.sorter()` before printing, and every output format
orders by `Violation::position().start`.

## What there is to decide

Whether a rule owes its caller source order at all. The sort in `Command::check`
is what the command needs; a library caller reading `RuleLike::check` gets
whatever the walk produced, which was source-ordered for every rule until a rule
started measuring more than one block of a container. Nothing tests it either way.

Tied to #425, which asks whether the library surface is public API before the next
release: if it is, this is part of what the surface promises.

Raised while working on #454, which is what made MD027 walk a quote's later
children.