docs: ramips-mt76x8: add Xiaomi Mi Router 4 model IDs

The model names are somewhat confusing. Add the model names that are
found on the device labels to make the variants easier to distinguish.

Based-on-patch-by: Andreas Ziegler <dev@andreas-ziegler.de>

docs/user/supported_devices.rst

@@ -540,9 +540,9 @@ ramips-mt76x8
 
 * Xiaomi
 
-  - Xiaomi Mi Router 4A (100M Edition)
-  - Xiaomi Mi Router 4A (100M International Edition)
-  - Xiaomi Mi Router 4C
+  - Xiaomi Mi Router 4A (100M Edition) - MIR4A
+  - Xiaomi Mi Router 4A (100M International Edition) - R4AC
+  - Xiaomi Mi Router 4C - R4CM
 
 rockchip-armv8
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