Document when references (both shared and exclusive) implement `RefUnwindSafe`

Location (URL)

https://doc.rust-lang.org/std/panic/trait.RefUnwindSafe.html

and

https://doc.rust-lang.org/std/primitive.reference.html

Summary

The prose of the Primitive Type reference page documents when various traits are implemented for references (both shared and exclusive). Notably it includes some auto traits, such as Send and Sync—but not others, such as Unpin, UnwindSafe or RefUnwindSafe.

Fortunately, the pages for those traits do document when they are implemented for reference types—except in the case of RefUnwindSafe! Indeed I cannot find it documented anywhere when RefUnwindSafe is implemented for &T or &mut T (I assume, in both cases, when T: RefUnwindSafe though I am not 100% certain).

At the very least, the RefUnwindSafe page should include the implementations for reference types.

Furthermore, for completeness, the prose of the Primitive Type reference page should probably include Unpin, UnwindSafe and RefUnwindSafe.

Start with the RefUnwindSafe and primitive reference documentation pages named in the issue, and compare their existing auto-trait prose with the relevant reference implementations. Verify the conditions for RefUnwindSafe on shared and exclusive references, then document them and add the missing Unpin, UnwindSafe, and RefUnwindSafe coverage to the primitive reference page. Done means both locations consistently explain these implementations.