It's not in the docs because its not a grav core features.. It's merely something that is supported by the Antimatter theme. Basically if you look in the templates/partials/base.html.twig of Antimatter you will see:
https://github.com/getgrav/grav-theme-antimatter/blob/develop/templates/partials/base.html.twig#L42-L42
<body id="top" class="{{ page.header.body_classes }}">
So basically this means that Antimatter theme, is looking for this page header and setting it on the <body> tag. This can be used by CSS to target any page.
Grav is super flexible allowing you to do things like this. This is not a specific configuration like you might see in other CMS platforms because it's totally free-form and unstructured.
Imagine it, then make it happen :) That's the Grav way :)