[$] On pages and folios

The kernel coverage here at LWN often touches on memory-management topicsand, as a result, tends to talk a lot about both pages and folios. As thefolio transition in the kernel has moved forward, it has often becomedifficult to decide which term to use in writing that is meant to be bothapproachable and technically correct. As this work continues, it will beincreasingly common to use "folio" rather than page. This article isintended to be a convenient reference for readers wanting to differentiatethe two terms or understand the state of this transition.