Add text about using `<figure>` for other things than images

[AndersEkl]

Add text about using <figure> for other things than images. Add text about class attributes table and other.

[oscarlcarlsson]

We had a look at captions for text-boxes: the topic that started this discussion. For text-boxes the other attribute doesn't seem to work out validation-wise. Our suggestion is to use <p class="caption" id="XXX> with an aria-labelledby-tag attribute in the text-box.

[AndersEkl]

@oscarlcarlsson Don't we need to update the validator anyway?

[AndersEkl]

There does not seem to be any formal xhtml problems with the intended markup. Validates fine using Oxygen's validation against the namespace.