Unclosed fenced code block doesn't always run until the end of the document #6

Section 4.5 (Fenced code blocks) says (emph. mine)

If the end of the document is reached and no closing code fence has been found, the code block contains all of the lines after the opening code fence.

but this isn't necessarily true; it may also be the end of the containing block that ends the fenced block. Example 137 is an instance of this:

> ```
foo
```

renders as

<blockquote>
<pre><code></code></pre>
</blockquote>
<p>foo</p>
<pre><code></code></pre>

I'm not making this a pull request since I'm not 100% on the best wording here.