wp: doc: add linking to docs

these don't work perfectly.

extendr-macros/src/extendr_impl.rs

@@ -36,6 +36,7 @@ use crate::{extendr_options::ExtendrOptions, wrappers};
 ///     fn aux_func;
 /// }
 /// ```
+#[doc(alias = "extendr-impl")]
 pub fn extendr_impl(mut item_impl: ItemImpl, opts: &ExtendrOptions) -> TokenStream {
     // Only `impl name { }` allowed
     if item_impl.defaultness.is_some() {

extendr-macros/src/lib.rs

@@ -75,6 +75,19 @@ use proc_macro::TokenStream;
 use quote::quote;
 use syn::{parse_macro_input, Item};
 
+
+/// The `#[extendr]`-macro may be placed on three items
+/// 
+/// - `fn` for wrapped rust-functions, see [`extendr-fn`]
+/// - `impl`-blocks, see [`extendr-impl`]
+/// - `Rust enum`s as R factors, see [`extendr-enum`]
+/// 
+/// [`extendr-fn`]: extendr-macros/extendr_function/fn.extendr_function.html
+/// [`extendr-impl`]: extendr_impl/fn.extendr_impl.html
+/// [`extendr-enum`]: extendr_enum/fn.extendr_enum.html
+/// 
+/// There is also [extendr_module!], which is used for defining what rust
+/// wrapped items should be visible to the surrounding R-package.
 #[proc_macro_attribute]
 pub fn extendr(attr: TokenStream, item: TokenStream) -> TokenStream {
     let item = parse_macro_input!(item as Item);