document -Zrandomize-layout in the unstable book

src/doc/unstable-book/src/compiler-flags/randomize-layout.md

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+# `randomize-layout`
+
+The tracking issue for this feature is: [#106764](https://github.com/rust-lang/rust/issues/106764).
+
+------------------------
+
+The `-Zrandomize-layout` flag changes the layout algorithm for `repr(Rust)` types defined in the current crate from its normal
+optimization goals to pseudorandomly rearranging fields within the degrees of freedom provided by the largely unspecified
+default representation. This also affects type sizes and padding.
+Downstream intantiations of generic types defined in a crate with randomization enabled will also be randomized.
+
+It can be used to find unsafe code that accidentally relies on unspecified behavior.
+
+Randomization is not guaranteed to use a different permutation for each compilation session.
+`-Zlayout-seed=<u64>` can be used to supply additional entropy.
+
+Randomization only approximates the intended freedom of repr(Rust). Sometimes two distinct types may still consistently
+result in the same layout due to limitations of the current implementation. Randomization may become
+more aggressive over time as our coverage of the available degrees of freedoms improves.
+Corollary: Randomization is not a safety oracle. Two struct layouts being observably the same under different layout seeds
+on the current compiler version does not guarantee that future compiler versions won't give them distinct layouts.
+
+Randomization may also become less aggressive in the future if additional guarantees get added to the default layout.