Suggestion for pagination/"before" documentation: clarify that only js front matter is supported

In the documentation for the "before" callback for pagination, it might be worth making explicit that this will only work for "js" front matter:

https://github.com/11ty/11ty-website/blob/c3fd39ccf04083f7ecf8a183aa201af84b8da52b/src/docs/pagination.md?plain=1#L452-L473

It may not be obvious to a reader that the "before" value of the front-matter here is a literal javascript function (rather than, say, a string that gets evaluated as javascript), and that therefore no other front-matter format besides "js" will work. (At least, I assume such is the case, from experimenting a little.)

It's true that a reader can perhaps infer this from the fact that the other examples on the page don't use js and this one does, or work it out with some trial and error: but people often read documentation in a hurry, so I think it's worth making this explicit.

Open src/docs/pagination.md at the linked lines and read the documentation for the “before” pagination callback alongside its front matter examples. Clarify that the value is a literal JavaScript function supported only with js front matter, then verify the wording remains consistent with the surrounding examples.