Explain that long options are matched based on a unique prefix rather than

requiring the whole option to be typed out.

This closes SF bug #126863.

Doc/lib/libgetopt.tex

@@ -29,7 +29,14 @@ recognize, with options that require an argument followed by a colon
 names of the long options which should be supported.  The leading
 \code{'-}\code{-'} characters should not be included in the option
 name.  Long options which require an argument should be followed by an
-equal sign (\character{=}).
+equal sign (\character{=}).  To accept only long options,
+\var{options} should be an empty string.  Long options on the command
+line can be recognized so long as they provide a prefix of the option
+name that matches exactly one of the accepted options.  For example,
+it \var{long_options} is \code{['foo', 'frob']}, the option
+\longprogramopt{fo} will match as \longprogramopt{foo}, but
+\longprogramopt{f} will not match uniquely, so \exception{GetoptError}
+will be raised.
 
 The return value consists of two elements: the first is a list of
 \code{(\var{option}, \var{value})} pairs; the second is the list of