Patch #781126: Fix markup for add_fallback. Will backport to 2.3.

Doc/lib/libgettext.tex

@@ -216,7 +216,7 @@ you have an unsupported message catalog file format, you should
 override this method to parse your format.
 \end{methoddesc}
 
-\begin{methoddesc}{NullTranslations}{add_fallback}{fallback}
+\begin{methoddesc}[NullTranslations]{add_fallback}{fallback}
 Add \var{fallback} as the fallback object for the current translation
 object. A translation object should consult the fallback if it cannot
 provide a translation for a given message.