Expand CloneToUninit documentation.

[kpreid]

• Clarify relationship to dyn after Make CloneToUninit dyn-compatible #133003.
• Add an example of using it with dyn as Make CloneToUninit dyn-compatible #133003 enabled.
• Replace parameter name dst with dest to avoid confusion between abbreviations for “DeSTination” and “Dynamically-Sized Type”.
• Add an example of implementing it.
• Add links to Rust Reference for the mentioned concepts.
• Mention that its method should rarely be called.
• Various small corrections.

Please review the unsafe code closely, as I am not an expert in the best possible ways to express these operations. (It might also be better to omit the implementation example entirely.)

cc @zachs18 #126799