I tested 1.8.22 and the captions (specified in the #img1 caption=’this is a caption’) is placed in the text via the HREF Code, and in the ALT parameter of the IMG tag. Here is what is generated (for lurkers) (URLs are santitized a bit, along with some extra line breaks for readability)
<a class="gallery_colorbox cboxElement" href="https://www.mysite.com/subsite/wp-content/uploads/sites/2/2016/12/storm-tree-3.jpg" data-featherlight="image">
<img class=" size-medium wp-image-83 alignleft "
src="https://www.mysite.com/subsite/wp-content/uploads/sites/2/2016/12/storm-tree-3-300x200.jpg"
alt="this is image 1"
srcset="https://www.mysite.com/subsite/wp-content/uploads/sites/2/2016
/12/storm-tree-3-300x200.jpg 300w, https://www.mysite.com/subsite/wp-content/uploads/sites/2/2016/12/storm-tree-3-768x512.jpg 768w, https://www.mysite.com/subsite/wp-content/uploads/sites/2/2016/12/storm-tree-3-676x450.jpg 676w, https://www.mysite.com/subsite/wp-content/uploads/sites/2/2016/12/storm-tree-3.jpg 821w"
sizes="(max-width: 300px) 100vw, 300px" width="300px" height="200px">
[Picture: this is image 1]
</a>
The caption, however, is not placed in the metadata for the picture (the info you see on the right when you click on a picture in the Media screen).
It would seem that having the ‘caption’ value in the ‘caption’ metadata would be the intent of the ‘caption’ code. That way, a theme could use the caption-metadata to put the caption of the image below the image, rather than as the HREF text.