doc: clarify kernel configuration usage in our repo

We keep in the repo the kernel configurations from which we build the
kernel images we use in our CI pipelines. However, we don't mention what
kernel sources we use to build these kernel images.

Clarify in the documentation that these configurations are meant to be
used with kernels provided by Amazon Linux and that we don't guarantee
that they can be used to build kernels from upstream Linux.

Signed-off-by: Babis Chalios <[email protected]>

Author: bchalios

docs/kernel-policy.md

@@ -37,6 +37,19 @@ The guest kernel configs used in our validation pipelines can be found
 [here](../resources/guest_configs/) while a breakdown of the relevant guest
 kernel modules can be found in the next section.
 
+We use these configurations to build microVM-specific kernels vended by Amazon
+Linux. microVM kernel source code is published in the Amazon Linux
+[linux repo](https://github.com/amazonlinux/linux) under tags in the form of
+`microvm-kernel-*`, e.g. 6.1.128-3.201.amazn2023 kernel can be found
+[here](https://github.com/amazonlinux/linux/tree/microvm-kernel-6.1.128-3.201.amzn2023).
+These kernels may have diverged from the equivalent mainline versions, as we
+often backport patches that we require for supporting Firecracker features not
+present in the kernel versions we officially support. As a result, kernel
+configurations found in this repo should be used to build exclusively the
+aforementioned Amazon Linux kernels. We do not guarantee that using these
+configurations to build upstream kernels, will work or produce usable kernel
+images.
+
 ## Guest kernel configuration items
 
 The configuration items that may be relevant for Firecracker are: