troubleshooting/upgrade: leave some place for other cases

Signed-off-by: Yann Dirson <yann.dirson@vates.tech>

Author: ydirson

docs/troubleshooting/installation-upgrade.md

@@ -32,9 +32,11 @@ The target installation partition is mounted in `/tmp/root`.
 ## The ISO installer does not offer to upgrade the existing install (XCP-ng or XenServer)
 
 :::note
-This section details how to deal with the most frequent cause for the installer not detecting your current installation. There can be other, rarer cases which are not detailed here.
+This section details how to deal with the most frequent causes for the installer not detecting your current installation. There can be other, rarer cases which are not detailed here.
 :::
 
+### UEFI/BIOS mismatch
+
 For the installer to detect your current install (XCP-ng or XenServer), the ISO must be booted in the same firmware context.
 The difference is how you pick the drive at boot. This applies to physical and virtual CD/DVD/USB drives.
 
@@ -55,7 +57,7 @@ Virtual CDROM Device         <<<<<< This one is the same device, in legacy BIOS
 ---------------------------------
 ```
 
-### How to check if a running install is using UEFI or legacy BIOS?
+#### How to check if a running install is using UEFI or legacy BIOS?
 
 On the host, run `efibootmgr`.
 
@@ -70,4 +72,4 @@ The main log file is `/var/log/installer/install-log`.
 
 ## Debugging the installer
 
-You can [build your own installer](../../project/development-process/ISO-modification).
+You can [build your own installer](../../project/development-process/ISO-modification).