doc: clarify cross-kernel snapshot compatibility

Update the explanation text to refer to the "working?" column instead of
some mystical "**all**", which doesnt exist. Also expand a bit on the
explanation to make it clearer what works and what doesnt.

Closes firecracker-microvm#4912
Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

docs/snapshotting/snapshot-support.md

@@ -655,11 +655,17 @@ snapshots [are not supported](./versioning.md#cpu-model)
 The tables below reflect the snapshot compatibility observed on the AWS
 instances we support.
 
-**all** means all currently supported Intel/AMD/ARM metal instances (m6g, m7g,
-m5n, c5n, m6i, m6a). It does not mean cross-instance, i.e. a snapshot taken on
-m6i won't work on an m6g instance.
+The *working?* column indicates whether snapshots taken on the host kernel
+version specified in the *taken* column can be restored on a host _of the same
+hardware kind_ running the kernel version specified in the *restored* column:
+E.g. a snapshot taken on a m6i.metal (Intel IceLake) instance with a 5.10 host
+kernel can be restored on a m6i.metal instance with a 6.1 host kernel, but _not_
+on a m5n.metal (Intel CascadeLake) instance with a 6.1 host kernel. **x86_64**
+means all currently supported Intel/AMD metal instances (m5n, c5n, m6i, m6a),
+while **aarch64** mean all currently support ARM metal instances (m6g, m7g).
 
 | *CPU family* | *taken on host kernel* | *restored on host kernel* | *working?* |
 | ------------ | ---------------------- | ------------------------- | ---------- |
 | **x86_64**   | 5.10                   | 6.1                       | Y          |
 | **x86_64**   | 6.1                    | 5.10                      | N          |
+| **aarch64**  | any                    | any                       | N          |