Fill-in missing argspec.

Doc/lib/libfuncs.tex

@@ -683,7 +683,7 @@ class C:
   \versionchanged[Added support for the optional \var{key} argument]{2.5}
 \end{funcdesc}
 
-\begin{funcdesc}{min}{s\optional{, args...}}
+\begin{funcdesc}{min}{s\optional{, args...}\optional{key}}
   With a single argument \var{s}, return the smallest item of a
   non-empty sequence (such as a string, tuple or list).  With more
   than one argument, return the smallest of the arguments.