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

usr.sbin/bsdinstall/bsdinstall.8

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 21, 2019
+.Dd February 6, 2020
 .Dt BSDINSTALL 8
 .Os
 .Sh NAME
@@ -118,6 +118,8 @@ is set, also configures the network interfaces of the current system to match.
 Provides the installer's interactive guided disk partitioner for single-disk
 installations.
 Defaults to UFS.
+.It Cm bootconfig
+Detects an appropriate partition and installs UEFI boot loader files.
 .It Cm zfsboot
 Provides a ZFS-only automatic interactive disk partitioner.
 Creates a single
@@ -266,6 +268,15 @@ Default:
 .It Ev DISTRIBUTIONS
 The set of distributions to install, e.g., "base.txz kernel.txz ports.txz".
 Default: unset
+.It Ev PARTITIONS
+The partitioning of the disk onto which the system is being installed.
+See
+.Cm scriptedpart
+of
+the
+.Sx TARGETS
+section for format details.
+Default: unset
 .It Ev BSDINSTALL_DISTDIR
 The directory in which the distribution files can be found (or to which they
 should be downloaded).
@@ -443,6 +454,7 @@ PARTITIONS=ada0
 DISTRIBUTIONS="kernel.txz base.txz"
 
 #!/bin/sh
+gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 ada0
 sysrc ifconfig_em0=DHCP
 sysrc sshd_enable=YES
 pkg install puppet