[PATCH] docs(user_manual): add machine kernel support section

["srinuvasan.a via isar-users" <isar-users@googlegroups.com>]

From: Srinuvasan A <srinuvasan.a@siemens.com>

Update the per-kernel modules build section.
Document that setting `KERNEL_NAMES` is sufficient to build
modules for multiple supported kernels.

Signed-off-by: Srinuvasan A <srinuvasan.a@siemens.com>
---
 doc/user_manual.md | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/doc/user_manual.md b/doc/user_manual.md
index a9348d26..2a9aca42 100644
--- a/doc/user_manual.md
+++ b/doc/user_manual.md
@@ -601,6 +601,21 @@ To add new machine user should perform the following steps:
 
 ---
 
+### Kernel Support
+
+A machine can be configured to support a specific kernel recipe by setting the `KERNEL_NAME` variable, and may be configured to support multiple kernelsby using the `KERNEL_NAMES` variable in addition. The latter is optional, and also enables generating packages like external kernel modules for all specified kernel variants.
+
+For example, in your machine configuration:
+
+```bitbake
+KERNEL_NAME = "armmp"
+KERNEL_NAMES = "armmp mainline"
+```
+
+When `KERNEL_NAMES` is set, recipes inheriting the `per-kernel` class will generate variants for each listed kernel. Installation of each must be explicitly handled in the image.
+
+---
+
 ## Add a New Image
 
 Image in Isar contains the following artifacts:
-- 
2.34.1

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