bhyve: Update usage and synopsis for the -k flag

Let's make it clear to users that -k is for configuration files.
Also, point to bhyve_config(5) in the paragraph describing the flag.

Reviewed by:	jhb
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D32467

usr.sbin/bhyve/bhyve.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd August 19, 2021
+.Dd October 13, 2021
 .Dt BHYVE 8
 .Os
 .Sh NAME
@@ -53,7 +53,7 @@
 .Ar port
 .Sm on
 .Oc
-.Op Fl k Ar file
+.Op Fl k Ar config_file
 .Oo Fl l
 .Sm off
 .Ar lpcdev Op Cm \&, Ar conf
@@ -168,7 +168,7 @@ Yield the virtual CPU thread when a HLT instruction is detected.
 If this option is not specified, virtual CPUs will use 100% of a host CPU.
 .It Fl h
 Print help message and exit.
-.It Fl k Ar file
+.It Fl k Ar config_file
 Set configuration variables from a simple, key-value config file.
 Each line of the config file is expected to consist of a config variable
 name, an equals sign
@@ -179,6 +179,9 @@ value.
 Blank lines and lines starting with
 .Sq #
 are ignored.
+See
+.Xr bhyve_config 5
+for more details.
 .It Fl l Cm help
 Print a list of supported LPC devices.
 .It Fl l Ar lpcdev Ns Op Cm \&, Ns Ar conf

usr.sbin/bhyve/bhyverun.c

@@ -224,7 +224,7 @@ usage(int code)
         fprintf(stderr,
 		"Usage: %s [-AaCDeHhPSuWwxY]\n"
 		"       %*s [-c [[cpus=]numcpus][,sockets=n][,cores=n][,threads=n]]\n"
-		"       %*s [-G port] [-k file] [-l lpc] [-m mem] [-o var=value]\n"
+		"       %*s [-G port] [-k config_file] [-l lpc] [-m mem] [-o var=value]\n"
 		"       %*s [-p vcpu:hostcpu] [-r file] [-s pci] [-U uuid] vmname\n"
 		"       -A: create ACPI tables\n"
 		"       -a: local apic is in xAPIC mode (deprecated)\n"