SF# 522426, add doc for common parameter for filecmp.cmpfiles()

Doc/lib/libfilecmp.tex

@@ -41,6 +41,7 @@ compared.  Files may be listed in \var{errors} because the user may
 lack permission to read them or many other reasons, but always that
 the comparison could not be done for some reason.
 
+The \var{common} parameter is a list of file names found in both directories.
 The \var{shallow} and \var{use_statcache} parameters have the same
 meanings and default values as for \function{filecmp.cmp()}.
 \end{funcdesc}