A good candidate to introduce longdesc might be the diagram example.

We have now,
[[
In the following example we have an image of a pie chart, with text in the alt attribute representing the data shown in the pie chart
]]

However, since Web browsers don't display alt text if it's too long to fit in the space allocated for the image (assuming width and height are supplied, eaither as attributes or from CSS, and default overflow rules), and not all browsers will even wrap alt text if it's too wide, we can't rely on being able to describe a pie chart in an alt attribute.

In any case even if we did, it would be extraordinarily irritating to have to listen to the long image description every time.

So I suggest.
[[
In the following example we have an image of a pie chart, with text in the alt attribute explaining that it is a pie chart, and the longdesc attribute giving a link to a full explanation with the colours, percentages and proportions. Note that a useful alternative representation of a pie chart might be to include a table of the same data in the document, and to use the ARIA describedby attribute to point to it, but that technique does not work for most diagrams, and a detailed description of an image (it's a circle with...) does not belong in the document itself for most readers.

<img src="piechart.png" alt="Pie chart: Browser Share" longdesc="browsershare.html">
]]
I think the rest of this example is OK, although use of describedat would help it considerably.

We believe that Liam will propose changes to HTML at some point, based on the work on the "alt text guidance" in the HTML a11y TF