Add advice to AT UA about presentation of <img> no alt

[travisleithead]

Opened because I also couldn't figure out what it meant, and better let @stevefaulkner clarify :)
See: https://www.w3.org/Bugs/Public/show_bug.cgi?id=23939

http://w3c.github.io/html/semantics-embedded-content.html#elementdef-img

[stevefaulkner]

I remembered what this was about. When an img without an alt is within a <figure> with a non-empty <figcaption> screen reader software should announce the presence of the image, as in its role "graphic" or whatever the particular AT uses to identify the image. This advice is to encourage AT to present users with some meaningful info about an image (its presence + the caption text contained within the figcaption) when no alt is provided, instead of a) ignoring the presence of the image or b) announcing the file path or somesuch.