Consider allowing fig-desc aside on image-series figure element

[martinpub]

We had a title with a generic caption for an image series, and then individually labelled figure images inside that. Our image description production followed the same format, but was unable to add a fig-desc aside on the series level, as it seems disallowed. As a work-around, the full image description was put in the first image of the image series, but that's not optimal.

I think we should consider allowing fig-desc in image series.

Currently, fig-desc is only introduced in the guidelines as a container for text extractions, but perhaps a revision could clarify that this is also the target element for image descriptions, even if this production would be outside the scope of the EPUB producers.

Not superhigh priority, just wanted to log this for now and see what you think.