chore: add comments for detect

Signed-off-by: usamoi <[email protected]>

crates/detect/src/lib.rs

@@ -1 +1,49 @@
+/// Function multiversioning attribute macros for `pgvecto.rs`.
+///
+/// ```rust
+/// #![feature(doc_cfg)]
+/// 
+/// #[cfg(any(target_arch = "x86_64", doc))]
+/// #[doc(cfg(target_arch = "x86_64"))]
+/// #[detect::target_cpu(enable = "v3")]
+/// unsafe fn g_v3(x: &[u32]) -> u32 {
+///     todo!()
+/// }
+///
+/// #[cfg(all(target_arch = "x86_64", test))]
+/// #[test)]
+/// fn g_v3_test() {
+///     const EPSILON: F32 = F32(1e-5);
+///     detect::init();
+///     if !detect::v3::detect() {
+///         println!("test {} ... skipped (v3)", module_path!());
+///         return;
+///     }
+///     let x = vec![0u32; 400];
+///     x.fill_with(|| rand::random());
+///     let specialized = unsafe { g_v3(&x) };
+///     let fallback = unsafe { g_fallback(&x) };
+///     assert!(
+///         (specialized - fallback).abs() < EPSILON,
+///         "specialized = {specialized}, fallback = {fallback}."
+///     );
+/// }
+///
+/// // It generates x86_64/v3, x86_64/v2, aarch64/neon and fallback versions of this function.
+/// // It takes advantage of `g_v4` as x86_64/v4 version of this function.
+/// // It exposes the fallback version with the name "g_fallback".
+/// #[detect::multiversion(v3 = import, v2, neon, fallback = export)]
+/// fn g(x: &[u32]) -> u32 {
+///     let mut result = 0_u32;
+///     for v in x {
+///         result = result.wrapping_add(*v);
+///     }
+///     result
+/// }
+/// ```
+pub use detect_macros::multiversion;
+
+/// This macros allows you to enable a set of features by target cpu names.
+pub use detect_macros::target_cpu;
+
 detect_macros::main!();

crates/detect_macros/src/lib.rs

@@ -307,8 +307,6 @@ pub fn main(_: proc_macro::TokenStream) -> proc_macro::TokenStream {
         });
     }
     quote::quote! {
-        pub use detect_macros::multiversion;
-        pub use detect_macros::target_cpu;
         #modules
         pub fn init() {
             #init