Where to put <desc> tags

In what appear to be the two main documents from W3C on accessibility in SVG [1,2], there seem to be slightly unclear advice in where to put <desc> tags.

In some of the examples, the <desc> precedes the group or object it is meant to describe:
<desc>this is a red rectangle meant to portray a button</desc>
<rect fill="red" />

In others, it is embedded as a child of the object it describes:

<rect fill="red" >
<desc>this is a red rectangle meant to portray a button</desc>
</rect>

The latter would seem to be a better approach since the <desc> is rather yoked to the thing it describes, but how are search engines and screen readers likely to deal with it? 

Is there another place where this is discussed, or am I missing something in my reading of these? What is the best advice for authors?

thanks
David
[1] (dated 2000) http://www.w3.org/TR/SVG-access/ 
{2] (dated 2008) http://www.w3.org/TR/SVG/access.html#SVGAccessibilityGuidelines