time complexity for push_within_capacity

Author: 20jasper

library/alloc/src/vec/mod.rs

@@ -1966,6 +1966,10 @@ 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> {