Added a synopsis.

Doc/lib/libzlib.tex

 % XXX The module has been extended (by Jeremy) but this documentation
-% hasn't been updated yet
+% hasn't been updated completely.
 
 \section{Built-in Module \module{zlib}}
 \declaremodule{builtin}{zlib}
 
-\modulesynopsis{None}
+\modulesynopsis{Low-level interface to compression and decompression
+routines compatible with \program{gzip}.}
 
 
 For applications that require data compression, the functions in this