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.
diff --git a/Doc/lib/libgetopt.tex b/Doc/lib/libgetopt.tex
index 6731c32..563460b 100644
--- a/Doc/lib/libgetopt.tex
+++ b/Doc/lib/libgetopt.tex
@@ -29,7 +29,14 @@
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