Currently, the docs for relative length¹ and ratio² are particularly simple. While the examples are helpful, I think it could be useful to mention that:

1. Ratios are usually (but not always) used as part of relative lengths (and thus have a link from ratio to relative length);
2. Ratios in relative lengths are usually (but not always) calculated in relation to the innermost container's width or height (depending on what is being set), or in relation to the page's width or height itself when there is no such container, however within margins.
3. However, within your own code, you can use ratios and relative lengths as you'd like. Most importantly, you can multiply a ratio by any number or length to obtain a new proportionally adjusted value.

Related to point 2, it could be useful to mention somewhere as well that, if one wants to apply lengths relative to the whole page's width and height, not being restricted to margins, one has to either create a page with 0pt margins (e.g. #page(margin: 0pt)[content using 100% as the full page width and height]) or to add their desired content to the current page's background (with #set page(background: [content using 100% as the full page width and height])).