Fix error in push docs

Copying is O(n)—not the memory allocation

Author: 20jasper

library/alloc/src/vec/mod.rs

@@ -1921,10 +1921,11 @@ impl<T, A: Allocator> Vec<T, A> {
     ///
     /// # Time complexity
     ///
-    /// Takes amortized *O*(1) time. If the vector's length would exceed its capacity after
-    /// the push, *O*(*capacity*) space is allocated, doubling the capacity and
-    /// taking *O*(*capacity*) time. This expensive operation is offset by the
-    /// *capacity* *O*(1) insertions it allows.
+    /// Takes amortized *O*(1) time. If the vector's length would exceed its
+    /// capacity after the push, the capacity is doubled by allocating
+    /// *O*(*capacity*) space, then *O*(*capacity*) time to copy the vector's
+    /// elements. 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")]