MFC 357638:

Approved by:	bcr (mentor)

Improve documentation of bootconfig and PARTITIONS

- Mention bootconfig target in TARGETS section.
- Document PARTITIONS variable, which is only mentioned in the examples,
  but doesn't have its own point.

Submitted by:	arrowd@
Reviewed by:	bcr
Approved by:	bcr (mentor)
Differential Revision:	https://reviews.freebsd.org/D22927