Update troubleshooting on boot mode mismatch

Observed on iLO 5: boot mode cannot be changed via Redfish.

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

Author: dtantsur

docs/user-guide/src/troubleshooting.md

@@ -128,10 +128,7 @@ Events:
   Normal  InspectionError     7m12s  metal3-baremetal-controller  timeout reached while inspecting the node
 ```
 
-### Incompatible configuration
-
-This can occur when attempting to use virtual media or UEFI on hardware that
-does not support it. The error will show in status and in events.
+### Error setting boot mode
 
 Example `kubectl get bmh -A`:
 
@@ -148,6 +145,20 @@ Redfish exception occurred. Error: Setting boot mode to bios failed for node cee
 https://192.168.111.1:8000/redfish/v1/Systems/... returned code 500.
 ```
 
+There are two possible causes:
+
+- Hardware does not support the requested boot mode. Really old machines may
+  not support UEFI, while really new machines gradually start phasing out
+  legacy boot.
+- Hardware does not support switching between boot modes using Redfish API.
+  This is the case for some iLO 5 machines.
+
+You have options:
+
+- Log into the BMC's UI and change the boot mode to the preferred value.
+- Update your BareMetalHost definition with the `bootMode` value that matches
+  what your hardware supports.
+
 ## Provisioning errors
 
 Errors during provisioning will be visible when listing the BareMetalHosts: