Reword and expand `# Invariant` section on `MiscDeviceRegistration`

[onestacked]

Currently the documentation on MiscDeviceRegistration:

/// A registration of a miscdevice.
///
/// # Invariants
///
/// `inner` is a registered misc device.

This invariant is not very precise, so reword and expand it, defining what it means for it to be "registered".

Originally Reported-by: Benno Lossin <lossin@kernel.org> in one of my Patch series.

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[Shankari02]

Hello,

I would want to take up this issue. Will send the patch to the mailing-list for review as per the prescribed format.