Document the contextlib module.
diff --git a/Doc/lib/lib.tex b/Doc/lib/lib.tex
index b588048..eac35de 100644
--- a/Doc/lib/lib.tex
+++ b/Doc/lib/lib.tex
@@ -371,6 +371,7 @@
 \input{libbltin}                % really __builtin__
 \input{libmain}                 % really __main__
 \input{libwarnings}
+\input{libcontextlib}
 \input{libatexit}
 \input{libtraceback}
 \input{libfuture}               % really __future__