Update stable/11 from 11.1-STABLE to 11.2-PRERELEASE, marking the
official start of the code slush.

Set the default mdoc(7) version to 11.2, and update the clang(1)
TARGET_TRIPLE to reflect 11.2. While here, add missing FreeBSD
major versions to mdoc(7).

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