Fix broken links

Author: JustForFun88

src/map.rs

@@ -944,6 +944,8 @@ impl<K, V, S, A: Allocator> HashMap<K, V, S, A> {
     ///
     /// Keeps the allocated memory for reuse.
     ///
+    /// [`retain()`]: HashMap::retain
+    ///
     /// # Examples
     ///
     /// ```

src/raw/mod.rs

@@ -910,6 +910,8 @@ impl<T, A: Allocator> RawTable<T, A> {
     ///
     /// The caller must ensure that the `RawTable` outlives the returned [`NonNull<T>`],
     /// otherwise using it may result in [`undefined behavior`].
+    ///
+    /// [`undefined behavior`]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     #[inline]
     pub fn data_end(&self) -> NonNull<T> {
         // SAFETY: `self.table.ctrl` is `NonNull`, so casting it is safe

src/set.rs

@@ -383,6 +383,8 @@ impl<T, S, A: Allocator> HashSet<T, S, A> {
     /// or the iteration short-circuits, then the remaining elements will be retained.
     /// Use [`retain()`] with a negated predicate if you do not need the returned iterator.
     ///
+    /// [`retain()`]: HashSet::retain
+    ///
     /// # Examples
     ///
     /// ```