[PATCH] doc/user_manual: mention EFI variable access and platform keyring for module signing

["'Cetin, Gokhan' via isar-users" <isar-users@googlegroups.com>]

Enrolling MOK or importing platform keys is not possible without access to EFI variables.

Signed-off-by: Gokhan Cetin <gokhan.cetin@siemens.com>
---
 doc/user_manual.md | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/doc/user_manual.md b/doc/user_manual.md
index 62d16c8c..bb8eb21b 100644
--- a/doc/user_manual.md
+++ b/doc/user_manual.md
@@ -1127,7 +1127,15 @@ modprobe example-module
 mokutil --import /etc/sb-mok-keys/MOK/MOK.der
 ```
 
-Use the previously definded password to enroll the key, then reboot.
+Use the previously defined password to enroll the key, then reboot.
+
+If EFI variable access is disabled on kernel (due to high latencies under RT kernel),
+enrolling will result in failure `EFI variables are not supported on this system`.
+EFI variable access can be enabled by passing `efi=runtime` kernel parameter.
+
+Similarly, in cases where EFI variables are not supported, the system will not be able
+to import the keys defined on the platform in the kernel platform keyring. This will also
+result in kernel modules not being verified if they are signed with one of these platform keys.
 
 **Boot self-signed image**:
 
-- 
2.39.2

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