Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

mdoc: move remaining sections into consistent order

This pertains mostly to FILES, HISTORY, EXIT STATUS and AUTHORS sections.

Found by: mdocml lint run
Reviewed by: ru

- loader(8) is not a part of boot(8) and is not its second stage.

Reported by: Yoshihiko Sarumaru <mistral at imasy.or.jp>
Reviewed by: trhodes
MFC-after: 3 days

Add the boot.config.5 manual page.

Reviewed by: keramida
Approved by: re@ (bmah)
PR: docs/112307