User guide: troubleshooting: describe the issue with forced deletion

This PR starts a troubleshooting FAQ for BMO. The first two entries
concern stuck cleaning and conflict after a forced deletion.

Signed-off-by: Dmitry Tantsur <[email protected]>

docs/user-guide/src/SUMMARY.md

@@ -28,6 +28,7 @@
       - [External Inspection](bmo/external_inspection.md)
       - [Live ISO](bmo/live-iso.md)
       - [Status Annotation](bmo/status_annotation.md)
+   - [Troubleshooting FAQ](bmo/troubleshooting.md)
 - [Ironic in Metal3](ironic/introduction.md)
    - [Install Ironic](ironic/ironic_installation.md)
    - [Ironic python agent](ironic/ironic-python-agent.md)

docs/user-guide/src/bmo/troubleshooting.md

@@ -0,0 +1,32 @@
+# Troubleshooting FAQ
+
+## Host is stuck in cleaning, how do I delete it?
+
+First and foremost, avoid using forced deletion, otherwise you'll have [a
+conflict](#mac-address-conflict-on-registration). If you don't care about disks
+being [cleaned](automated_cleaning.md), you can edit the BareMetalHost resource
+and disable cleaning:
+
+```yaml
+spec:
+  automatedCleaningMode: disabled
+```
+
+Alternatively, you can wait for 3 cleaning retries to finish. After that, the
+host will be deleted. If you do care about cleaning, you need to figure out why
+it does not finish.
+
+## MAC address conflict on registration
+
+If you force deletion of a host after registration, BareMetal Operator will not
+be able to delete the corresponding record from Ironic. If you try to enroll
+the same host again, you will see the following error:
+
+```text
+Normal  RegistrationError  4m36s  metal3-baremetal-controller  MAC address 11:22:33:44:55:66 conflicts with existing node namespace~name
+```
+
+Currently, the only way to get rid of this error is to re-create the Ironic's
+internal database. If your deployment uses SQLite (the default), it is enough
+to restart the pod with Ironic. If you use MariaDB, you need to restart its
+pod, clearing any persistent volumes.