extern blocks don't have doc comments

[RalfJung]

Fixes a warning I see when building this code:

warning: unused doc comment
   --> crates/core_simd/src/intrinsics.rs:21:1
    |
21  | / /// These intrinsics aren't linked directly from LLVM and are mostly undocumented, however they are
22  | | /// mostly lowered to the matching LLVM instructions by the compiler in a fairly straightforward manner.
23  | | /// The associated LLVM instruction or intrinsic is documented alongside each Rust intrinsic function.
    | |______________________________________________________________________________________________________^
24  | / extern "platform-intrinsic" {
25  | |     /// add/fadd
26  | |     pub(crate) fn simd_add<T>(x: T, y: T) -> T;
27  | |
...   |
150 | |     pub(crate) fn simd_select_bitmask<M, T>(m: M, yes: T, no: T) -> T;
151 | | }
    | |_- rustdoc does not generate documentation for extern block
    |
    = note: `#[warn(unused_doc_comments)]` on by default
    = help: use `//` for a plain comment

[RalfJung]

Or maybe this should be made part of the module level doc comment up top?

[workingjubilee]

Eh, this way is fine!