Rollup merge of #121262 - 20jasper:add-vector-time-complexity, r=cuviper

Add vector time complexity

Added time complexity for `Vec` methods `push`, `push_within_capacity`, `pop`, and `insert`.

<details>

<summary> Reference images </summary>

![`Vec::push` documentation](https://github.com/rust-lang/rust/assets/78604367/dc966bbd-e92e-45a6-af82-35afabfa79a9)

![`Vec::push_within_capacity` documentation](https://github.com/rust-lang/rust/assets/78604367/9aadaf48-46ed-4fad-bdd5-74b98a61f4bb)

![`Vec::pop` documentation](https://github.com/rust-lang/rust/assets/78604367/88ec0389-a346-4ea5-a3b7-17caf514dd8b)

![`Vec::insert` documentation](https://github.com/rust-lang/rust/assets/78604367/960c15c3-ef8e-4aa7-badc-35ce80f6f221)

</details>

I followed a convention to use `#Time complexity` that I found in [the `BinaryHeap` documentation](https://doc.rust-lang.org/std/collections/struct.BinaryHeap.html#time-complexity-1). Looking through the rest of standard library collections, there is not a consistent way to handle this.

[`Vec::swap_remove`](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.swap_remove) does not have a dedicated section for time complexity but does list it.

[`VecDeque::rotate_left`](https://doc.rust-lang.org/std/collections/struct.VecDeque.html#complexity) uses a `#complexity` heading.

library/alloc/src/vec/mod.rs

@@ -1490,6 +1490,12 @@ impl<T, A: Allocator> Vec<T, A> {
     /// vec.insert(4, 5);
     /// assert_eq!(vec, [1, 4, 2, 3, 5]);
     /// ```
+    ///
+    /// # Time complexity
+    ///
+    /// Takes *O*([`Vec::len`]) time. All items after the insertion index must be
+    /// shifted to the right. In the worst case, all elements are shifted when
+    /// the insertion index is 0.
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn insert(&mut self, index: usize, element: T) {
@@ -1913,6 +1919,13 @@ impl<T, A: Allocator> Vec<T, A> {
     /// vec.push(3);
     /// assert_eq!(vec, [1, 2, 3]);
     /// ```
+    ///
+    /// # Time complexity
+    ///
+    /// Takes amortized *O*(1) time. If the vector's length would exceed its
+    /// capacity after the push, *O*(*capacity*) time is taken to copy the
+    /// vector's elements to a larger allocation. This expensive operation is
+    /// offset by the *capacity* *O*(1) insertions it allows.
     #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -1961,6 +1974,10 @@ impl<T, A: Allocator> Vec<T, A> {
     /// }
     /// assert_eq!(from_iter_fallible(0..100), Ok(Vec::from_iter(0..100)));
     /// ```
+    ///
+    /// # Time complexity
+    ///
+    /// Takes *O*(1) time.
     #[inline]
     #[unstable(feature = "vec_push_within_capacity", issue = "100486")]
     pub fn push_within_capacity(&mut self, value: T) -> Result<(), T> {
@@ -1990,6 +2007,10 @@ impl<T, A: Allocator> Vec<T, A> {
     /// assert_eq!(vec.pop(), Some(3));
     /// assert_eq!(vec, [1, 2]);
     /// ```
+    ///
+    /// # Time complexity
+    ///
+    /// Takes *O*(1) time.
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn pop(&mut self) -> Option<T> {