Guide: advice for images from source is out of date

[sean-fitzpatrick]

This section of the guide provides advice on including images from source code (Asymptote, TikZ).

It still recomments putting an xml:id on the <image> element, rather than a label on the <asymptote> or <latex-image>.

Yes I can make a PR to fix it, but I just noticed this while in the middle of something else and don't want to forget.

[sean-fitzpatrick]

And now the PR is at #2555