Document additional constructs caught by the unsafe_code lint

[scottmcm]

Today, the unsafe_code lint documentation https://doc.rust-lang.org/nightly/rustc/lints/listing/allowed-by-default.html#unsafe-code only mentions that it catches unsafe blocks.

But it actually catches more than that, for example

error: declaration of a `no_mangle` function
 --> src/lib.rs:3:1
  |
3 | #[no_mangle]
  | ^^^^^^^^^^^^
  |
  = note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
note: the lint level is defined here
 --> src/lib.rs:1:11
  |
1 | #![forbid(unsafe_code)]
  |           ^^^^^^^^^^^

It would be nice for the documentation to include examples of those other things that the lint catches because they might result in unsoundness despite the unsafe token not appearing in them.

[eval-exec]

@rustbot claim

[eval-exec]

Do we need to indicate unsafe_code will emit no_mangle, export_name and link_section in the unsafe_code lint document?
https://github.com/rust-lang/rust/blob/5783e699b3a8a2fbef7294f200c16a2614c3e7ed/compiler/rustc_lint/src/builtin.rs#L375-L410