doc: Limitations: add notes about no support for validation/validation-set assertions

[tonyespy]

Please add an additional bullet point under Limitations which states that validation and validation-set assertions are not supported. Customers requiring finer granularity of control over which snap revisions are refreshed on devices connected to an offline proxy should either use Snap revision overrides or use a device management system such as Landscape.

The existing limitations bullet also includes the following item which seems pretty nonsensical as currently written:

Generic Device registration. Serial Vault can be used to register custom model devices.

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Document: en/airgap.md