Fix emphasis in already emphasized paragraph.

Markdown (or at least Github's parser) seems to struggle with nested emphasis markup.

- Both `*[…]*` and `**[…]**` are not parsed correctly.
- `<em>[…]</em>` is visually the same in this context.
- `<em style="font-style: normal;">[…]</em>` is still italic, and ugly.

So I went with `<strong>[…]</strong>`.

reStructuredText's [admonitions](http://docutils.sourceforge.net/docs/ref/rst/directives.html#admonitions) would nicely avoid this.