Fixing bug 405999 -- clarifying differences between Python's

getopt and GNU getopt -- Python is like classical UNIX getopt.

Doc/lib/libgetopt.tex

@@ -25,6 +25,10 @@ recognize, with options that require an argument followed by a colon
 (\character{:}; i.e., the same format that \UNIX{}
 \cfunction{getopt()} uses).
 
+\strong{Note:} Unlike GNU \cfunction{getopt()}, after a non-option
+argument, all further arguments are considered also non-options.
+This is similar to the way non-GNU \UNIX{} systems work.
+
 \var{long_options}, if specified, must be a list of strings with the
 names of the long options which should be supported.  The leading
 \code{'-}\code{-'} characters should not be included in the option