Clarify semantics around `uniffi_reexport_scaffolding!` macro

[mhammond]

Our docs talk about uniffi_reexport_scaffolding!, which links to rust-lang#50007, which is marked as "fixed". According to #1242, that was fixed in Rust 1.62, and we are way past that.

However, if I patch m-c to remove uniffi_reexport_scaffolding! I see undefined symbol errors for all our scaffolding functions. So it appears something remains necessary for symbols in other crates to be exported in the final library.

I bring this up because according to #2459, the code generated by this macro fails to build with Rust 2024 - instead of "fixing" that I was hoping we could just remove the capability entirely, but apparently we can not. Assume that's true, we should try and better document this work-around so we aren't blaming an issue that no longer exists.