doc: Offline store serial assertion note needs improvement

[tonyespy]

The Offline store (air-gapped mode) page includes a note that says:

Client devices have to be equipped with their serial assertions.

This is awkwardly worded and a bit vague...

This note also includes a link to the Obtaining serial assertions section of the Proxy's Configuring devices page.

Why is this problematic? Essentially what's being is suggested is to allow the devices to obtain a serial assertion via the Snap Store Proxy in online mode before air-gap mode is enabled, yet the air-gap page also includes a note that says air-gap mode should only be enabled on a proxy that hasn't yet had any devices connected!

There are two other possibilities:

1. The devices when first booted are connected to the Internet for long enough (note, the time to acquire a serial assertion is non-deterministic) to gain a serial from the Serial Vault.

2. The devices are connected to a second Snap Store Proxy in online mode for long enough that they acquire a serial assertion, then devices are redirected to a new Snap Store Proxy instance in air-gap mode.

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Document: en/airgap.md