Fix time complexity in BinaryHeap::peek_mut docs

Author: SkiFire13

library/alloc/src/collections/binary_heap.rs

@@ -399,7 +399,8 @@ impl<T: Ord> BinaryHeap<T> {
     ///
     /// # Time complexity
     ///
-    /// Cost is *O*(1) in the worst case.
+    /// If the item is modified then the worst case time complexity is *O*(log(*n*)),
+    /// otherwise it's *O*(1).
     #[stable(feature = "binary_heap_peek_mut", since = "1.12.0")]
     pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T>> {
         if self.is_empty() { None } else { Some(PeekMut { heap: self, sift: false }) }