Update: usage doc for heappushpop (GH-91451)

Lib/heapq.py

@@ -12,6 +12,8 @@
 item = heappop(heap) # pops the smallest item from the heap
 item = heap[0]       # smallest item on the heap without popping it
 heapify(x)           # transforms list into a heap, in-place, in linear time
+item = heappushpop(heap, item) # pushes a new item and then returns
+                               # the smallest item; the heap size is unchanged
 item = heapreplace(heap, item) # pops and returns smallest item, and adds
                                # new item; the heap size is unchanged