Rollup merge of #100872 - JanBeh:PR_vec_default_alloc_doc, r=fee1-dead

Add guarantee that Vec::default() does not alloc

Currently `Vec::new()` is guaranteed to not allocate until elements are pushed onto the `Vec`, but such a guarantee is missing for `Vec`'s implementation of `Default::default`.

This adds such a guarantee for `Vec::default()` to the API reference.

See also [this discussion on URLO](https://users.rust-lang.org/t/guarantee-that-vec-default-does-not-allocate/79903).

library/alloc/src/vec/mod.rs

@@ -2928,6 +2928,8 @@ unsafe impl<#[may_dangle] T, A: Allocator> Drop for Vec<T, A> {
 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
 impl<T> const Default for Vec<T> {
     /// Creates an empty `Vec<T>`.
+    ///
+    /// The vector will not allocate until elements are pushed onto it.
     fn default() -> Vec<T> {
         Vec::new()
     }