content: Clarify sort order with PAGE.Rotate

[jmooring]

https://deploy-preview-3408--gohugoio.netlify.app/methods/page/rotate/

This adds some detail (fallback) to the sort order, and provides an introductory sentence to each example.

[bep]

I think this is great and don't hesitate to merge it.

As a side note, the natural thing would be to write:

{{ range .Rotate "language" }}
    <h2><a href="{{ .RelPermalink }}">{{ .LinkTitle }}</a></h2>
{{ end }}

[jmooring]

the natural thing

I have no idea why I wrote it that way. In my defense, I'm pretty sure it was well after midnight.