dumpon(8): Add a SYSCTL VARIABLES section

Document debug.minidump and machdep.dump_retry_count sysctl variables.

PR:			259393
MFC after:		3 seconds
Reviewed by:		gbe (previous), markj, ziaee
Differential Revision:	https://reviews.freebsd.org/D32618

sbin/dumpon/dumpon.8

@@ -228,6 +228,20 @@ total amount of physical memory as reported by the
 .Va hw.physmem
 .Xr sysctl 8
 variable.
+.Sh SYSCTL VARIABLES
+The following
+.Xr sysctl 8
+variables can be used to modify or monitor the behavior of crash dumps:
+.Bl -tag -width "machdep.dump_retry_count"
+.It Va debug.minidump
+Set the type of kernel crash dump.
+Possible values are 0 for a full crash dump or 1 for a minidump.
+The default is minidump.
+.It Va machdep.dump_retry_count
+The maximum number of times dump will retry before giving up.
+The default value is 5.
+This sysctl is only supported on PowerPC and AMD64.
+.El
 .Sh IMPLEMENTATION NOTES
 Because the file system layer is already dead by the time a crash dump
 is taken, it is not possible to send crash dumps directly to a file.
@@ -377,6 +391,7 @@ needed.
 .Xr loader 8 ,
 .Xr rc 8 ,
 .Xr savecore 8 ,
+.Xr sysctl 8 ,
 .Xr swapon 8 ,
 .Xr panic 9
 .Sh HISTORY