Rollup merge of rust-lang#42151 - Wallacoloo:docfix_into_vec, r=frewsxcv

Mention Vec::into_boxed_slice in documentation of [T]::into_vec

This is a documentation fix.

`Vec::into_boxed_slice` and `[T]::into_vec` are inverses, so it makes sense to mention the other in their respective documentation for visibility. `Vec::into_boxed_slice` already mentions `[T]::into_vec`, but not the other way around until now.

Tagging @steveklabnik per his request.

Author: frewsxcv

src/libcollections/slice.rs

@@ -1410,6 +1410,9 @@ impl<T> [T] {
 
     /// Converts `self` into a vector without clones or allocation.
     ///
+    /// The resulting vector can be converted back into a box via
+    /// `Vec<T>`'s `into_boxed_slice` method.
+    ///
     /// # Examples
     ///
     /// ```