Documentation and clarification about "CKAN Flavored Markdown"

As stated on the [mailinglist](http://lists.okfn.org/pipermail/ckan-dev/2013-November/006175.html) and in some of the issues like #1270 , not all features of __Markdown__ are supported, especially regarding embedding native HTML code. For example, adding an Image with `<img>` does unfortunately not work.

The problem is that the CKAN editor links to the documentation [Markdown: Syntax](http://daringfireball.net/projects/markdown/syntax), which describes that HTML code like `<img>` works. So, we need to clarify which features are supported. There should at least be a documentation about "what does not work" compared to the linked documentation.