troubleshooting/operating-system: document kdump support over NFS

On RHCOS, the nfs module is explicitly excluded from the initramfs:
https://github.com/coreos/fedora-coreos-config/blob/de0416b8820cffa2ea3b7dbd9e95a9cc0ad9a42b/overlay.d/05core/usr/lib/dracut/dracut.conf.d/coreos-omits.conf#L7
Thus, setting an NFS target destination in kdump.conf will not work
because the kdump initramfs will not have the NFS module. Add
instruction on how to add the module to enable the functionality.

This may not be necessary if kdump auto-detect the required modules:
See rhkdump/kdump-utils#11

modules/troubleshooting-enabling-kdump-day-one.adoc

@@ -68,6 +68,18 @@ systemd:
 <4> If you want to change the contents of `/etc/sysconfig/kdump` from the default, include this section and modify the `inline` subsection accordingly.
 <5> For the `ppc64le` platform, replace `nr_cpus=1` with `maxcpus=1`, which is not supported on this platform.
 
+[NOTE]
+====
+To export the dumps to NFS targets, the nfs kernel module must be explicitely added to the configuration file:
+[source]
+---
+# /etc/kdump.conf contents:
+nfs penguin.example.com:/export/cores
+core_collector makedumpfile -l --message-level 7 -d 31
+extra_modules nfs
+---
+====
+
 . Use Butane to generate a machine config YAML file, `99-worker-kdump.yaml`, containing the configuration to be delivered to the nodes:
 +
 [source,terminal]