allocations: document that they can be read-only

[RalfJung]

Miri currently tracks "read-only" as an explicit flag on allocations that exists independent of provenance. It essentially corresponds to a read-only mapping in the page table.

Let's make this officially part of our model.
Cc @rust-lang/lang @rust-lang/opsem

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[RalfJung]

@rfcbot merge opsem

[rust-rfcbot]

@RalfJung has proposed to merge this. The next step is review by the rest of the tagged team members:

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• @RalfJung
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• @joshlf
• @saethlin

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