Rollup merge of rust-lang#105032 - HintringerFabian:improve_docs, r=JohnTitor

improve doc of into_boxed_slice and impl From<Vec<T>> for Box<[T]>

Improves description of `into_boxed_slice`, and adds example to `impl From<Vec<T>> for Box<[T]>`.
Fixes rust-lang#98908

Author: Yuki Okushi

library/alloc/src/vec/mod.rs

@@ -1070,7 +1070,8 @@ impl<T, A: Allocator> Vec<T, A> {
 
     /// Converts the vector into [`Box<[T]>`][owned slice].
     ///
-    /// Note that this will drop any excess capacity.
+    /// If the vector has excess capacity, its items will be moved into a
+    /// newly-allocated buffer with exactly the right capacity.
     ///
     /// [owned slice]: Box
     ///
@@ -3223,6 +3224,14 @@ impl<T, A: Allocator> From<Vec<T, A>> for Box<[T], A> {
     /// ```
     /// assert_eq!(Box::from(vec![1, 2, 3]), vec![1, 2, 3].into_boxed_slice());
     /// ```
+    ///
+    /// Any excess capacity is removed:
+    /// ```
+    /// let mut vec = Vec::with_capacity(10);
+    /// vec.extend([1, 2, 3]);
+    ///
+    /// assert_eq!(Box::from(vec), vec![1, 2, 3].into_boxed_slice());
+    /// ```
     fn from(v: Vec<T, A>) -> Self {
         v.into_boxed_slice()
     }