[markdown/en] clarify strikethrough text and fix alt-attribute claims

These are two super-small fixes:

1. Strikehrough text is only available in Github Flavored Markdown, so it shouldn't appear anywhere else but in the Github Flavored Markdown-section.

2. The `alt`-attribute on an `<img>` is *NOT* the text that is displayed on hover, it's the `title`-attribute that will be displayed.
```markdown
![foo](http://userserve-ak.last.fm/serve/126/1613535.jpg "bar")
```
will render as:
![foo](http://userserve-ak.last.fm/serve/126/1613535.jpg "bar")