Document new heapreplace() function.
diff --git a/Doc/lib/libheapq.tex b/Doc/lib/libheapq.tex
index d1aaaae..38c38d8 100644
--- a/Doc/lib/libheapq.tex
+++ b/Doc/lib/libheapq.tex
@@ -52,6 +52,15 @@
Transform list \var{x} into a heap, in-place, in linear time.
\end{funcdesc}
+\begin{funcdesc}{heapreplace}{heap, item}
+Pop and return the smallest item from the \var{heap}, and also push
+the new \var{item}. The heap size doesn't change.
+This is more efficient than \function{heappop()} followed
+by \function{heappush()}, and can be more appropriate when using
+a fixed-size heap. Note that the value returned may be larger
+than \var{item}! That constrains reasonable uses of this routine.
+\end{funcdesc}
+
Example of use:
\begin{verbatim}