boot(8): Add additional cross references

Provide hints to direct people towards gptzfsboot, zfsbootcfg, efibootmgr, etc

PR:		199103
MFC after:	2 weeks
Sponsored by:	Klara Inc.
Event:		July 2020 Bugathon

sbin/reboot/boot_i386.8

@@ -36,7 +36,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd June 7, 2020
+.Dd July 11, 2020
 .Dt BOOT 8 i386
 .Os
 .Sh NAME
@@ -330,11 +330,15 @@ requirement has not been adhered to.
 .Xr config 8 ,
 .Xr gpart 8 ,
 .Xr gptboot 8 ,
+.Xr gptzfsboot 8 ,
 .Xr halt 8 ,
 .Xr loader 8 ,
 .Xr nextboot 8 ,
 .Xr reboot 8 ,
 .Xr shutdown 8 ,
+.Xr zfsbootcfg 8 ,
+.Xr efibootmgr 8 ,
+.Xr efivar 8 ,
 .Xr uefi 8
 .Sh BUGS
 The bsdlabel format used by this version of