crashinfo: Add references to the gdb port/package

We intend to remove the obsolete GDB 6.1.1 from FreeBSD before FreeBSD 13.

Reviewed by	jhb
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D27447

usr.sbin/crashinfo/crashinfo.8

@@ -27,7 +27,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd April 19, 2017
+.Dd December 2, 2020
 .Dt CRASHINFO 8
 .Os
 .Sh NAME
@@ -85,6 +85,7 @@ it uses several utilities to analyze the core including
 .Xr pstat 8 ,
 and
 .Xr vmstat 8 .
+Note that kgdb must be installed from the devel/gdb port or gdb package.
 .Pp
 The options are as follows:
 .Bl -tag -width indent

usr.sbin/crashinfo/crashinfo.sh

@@ -177,6 +177,7 @@ fi
 find_gdb
 if [ -z "$GDB" ]; then
 	echo "Unable to find a kernel debugger."
+	echo "Please install the devel/gdb port or gdb package."
 	exit 1
 fi