Rollup merge of rust-lang#89210 - Takashiidobe:master, r=kennytm

Add missing time complexities to linked_list.rs

Most functions in LinkedList have time complexities in their description:
Like push front:

```
Adds an element first in the list.

This operation should compute in O(1) time.
```

Time complexities were missing for the following, so I've added them in this PR:

contains: O(n)
front: O(1)
front_mut: O(1)
back: O(1)
back_mut: O(1)

Author: workingjubilee

library/alloc/src/collections/linked_list.rs

@@ -631,6 +631,8 @@ impl<T> LinkedList<T> {
     /// Returns `true` if the `LinkedList` contains an element equal to the
     /// given value.
     ///
+    /// This operation should compute in *O*(*n*) time.
+    ///
     /// # Examples
     ///
     /// ```
@@ -656,6 +658,8 @@ impl<T> LinkedList<T> {
     /// Provides a reference to the front element, or `None` if the list is
     /// empty.
     ///
+    /// This operation should compute in *O*(1) time.
+    ///
     /// # Examples
     ///
     /// ```
@@ -676,6 +680,8 @@ impl<T> LinkedList<T> {
     /// Provides a mutable reference to the front element, or `None` if the list
     /// is empty.
     ///
+    /// This operation should compute in *O*(1) time.
+    ///
     /// # Examples
     ///
     /// ```
@@ -702,6 +708,8 @@ impl<T> LinkedList<T> {
     /// Provides a reference to the back element, or `None` if the list is
     /// empty.
     ///
+    /// This operation should compute in *O*(1) time.
+    ///
     /// # Examples
     ///
     /// ```
@@ -722,6 +730,8 @@ impl<T> LinkedList<T> {
     /// Provides a mutable reference to the back element, or `None` if the list
     /// is empty.
     ///
+    /// This operation should compute in *O*(1) time.
+    ///
     /// # Examples
     ///
     /// ```