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)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D22927