Rollup merge of #126118 - jan-ferdinand:docs_for_vec_set_len, r=the8472

docs: Mention `spare_capacity_mut()` in `Vec::set_len`

I recently went down a small rabbit hole when trying to identify safe use of `Vec::set_len`. The solution was `Vec::spare_capacity_mut`. I think the docs on `Vec::set_len` benefit from mentioning this method.

A possible counter-argument could be that the [clippy lint `uninit_vec`](https://rust-lang.github.io/rust-clippy/master/index.html#/uninit_vec) already nudges people in the right direction. However, I think a working example on `Vec::set_len` is still beneficial.

Happy to hear your thoughts on the matter. 😊

Author: jhpratt

library/alloc/src/vec/mod.rs

@@ -1824,7 +1824,10 @@ impl<T, A: Allocator> Vec<T, A> {
     ///
     /// # Examples
     ///
-    /// This method can be useful for situations in which the vector
+    /// See [`spare_capacity_mut()`] for an example with safe
+    /// initialization of capacity elements and use of this method.
+    ///
+    /// `set_len()` can be useful for situations in which the vector
     /// is serving as a buffer for other code, particularly over FFI:
     ///
     /// ```no_run
@@ -1884,6 +1887,8 @@ impl<T, A: Allocator> Vec<T, A> {
     ///
     /// Normally, here, one would use [`clear`] instead to correctly drop
     /// the contents and thus not leak memory.
+    ///
+    /// [`spare_capacity_mut()`]: Vec::spare_capacity_mut
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub unsafe fn set_len(&mut self, new_len: usize) {