Fix docs for alloc::realloc

[overlookmotel]

Fixes #108546.

Corrects the docs for alloc::realloc to bring the safety constraints into line with Layout::from_size_align_unchecked's constraints.

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[cuviper]

See #108546 (comment), this was deliberately left out of the isize::MAX limits for now...

[cuviper]

Closing to defer the decision to rust-lang/libs-team#61

[Amanieu]

We discussed this in the libs meeting and decided that this is the best way forward for GlobalAlloc. Adding a new method would cause too much churn and we plan on deprecating GlobalAlloc to replace it with Allocator everywhere.

@rfcbot fcp merge

[rfcbot]

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

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

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[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[Amanieu]

@bors r+ rollup

[bors]

📌 Commit 48e8126 has been approved by Amanieu

It is now in the queue for this repository.