[PATCH docs] bootloaders: add secure boot shim example

[Maximiliano Sandoval <m.sandoval@proxmox.com>]

On a system that was migrated from systemd-boot to grub2 with secure
boot, the entry grubx64.efi would be missing and the systemd-bootx64.efi
entry would be present. If the shimx64.efi entry is not mentioned, then
the docs would incorrectly imply that the system uses systemd-boot as a
bootloader.

We add the shimx64.efi entry to the docs to fill this gap.

Suggested-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
---

Notes:
    On my machine:
    
    $ efibootmgr -v
    Boot0000* Linux Boot Manager
    HD(2,GPT,[..])/File(\EFI\systemd\systemd-bootx64.efi) [..]
    [..]
    Boot0002* proxmox
    HD(2,GPT,[..])/File(\EFI\proxmox\shimx64.efi) [..]

 system-booting.adoc | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/system-booting.adoc b/system-booting.adoc
index da87697..c3de158 100644
--- a/system-booting.adoc
+++ b/system-booting.adoc
@@ -209,6 +209,13 @@ used in UEFI mode.
 Boot0005* proxmox	[...] File(\EFI\proxmox\grubx64.efi)
 ----
 
+If the output contains a line that looks similar to the following, GRUB is
+used in UEFI mode with Secure Boot enabled.
+
+----
+Boot0005* proxmox	[...] File(\EFI\proxmox\shimx64.efi)
+----
+
 If the output contains a line similar to the following, `systemd-boot` is used.
 
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-- 
2.47.3