Clarify preconditions of raw size/align methods

[Jules-Bertholet]

The documentation of the preconditions of mem::size_of_val_raw, mem::align_of_val_raw, and Layout::for_value_raw was overly conservative. This PR resolves that problem.

Discussion: #69835 (comment)

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[rust-highfive]

r? @m-ou-se

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[CAD97]

I think it'd perhaps be better worded as something around the shape of

Safety

This function is safe to call if the pointer is valid to reborrow as &T (in which case you could also call size_of_val). Otherwise, the following conditions must hold:

(existing bullet points)

This makes is clearer that "valid for reborrow" is always a sufficient precondition.

[m-ou-se]

The exception/note about reborrowing as &T now appears at both the top and bottom of the list of conditions, which is a bit confusing. Right now it's basically: "This function is safe to call if {A}. Otherwise: {list of conditions}. Otherwise, {A}."

Isn't just the note at the start (as suggested by CAD97) enough?

[JohnCSimon]

@Jules-Bertholet
Ping from triage - can you please address the reviewer's comments?

[Dylan-DPC]

Closing this as inactive. Feel free to open a new PR if you wish to continue these changes