diff --git a/Doc/lib/libgzip.tex b/Doc/lib/libgzip.tex
index ba3267cc672..3df75ce8bd5 100644
--- a/Doc/lib/libgzip.tex
+++ b/Doc/lib/libgzip.tex
@@ -11,7 +11,10 @@ with that used by the GNU compression program \program{gzip}.
 Accordingly, the \module{gzip} module provides the \class{GzipFile}
 class to read and write \program{gzip}-format files, automatically
 compressing or decompressing the data so it looks like an ordinary
-file object.
+file object.  Note that additional file formats which can be
+decompressed by the \program{gzip} and \program{gunzip} programs, such 
+as those produced by \program{compress} and \program{pack}, are not
+supported by this module.
 
 The module defines the following items:
 
@@ -34,10 +37,12 @@ to the filename of \var{fileobj}, if discernible; otherwise, it
 defaults to the empty string, and in this case the original filename
 is not included in the header.
 
-The \var{mode} argument can be either \code{'r'} or \code{'w'},
-depending on whether the file will be read or written.  The default is
-the mode of \var{fileobj} if discernible; otherwise, the default is
-\code{'r'}.
+The \var{mode} argument can be any of \code{'r'}, \code{'rb'},
+\code{'a'}, \code{'ab'}, \code{'w'}, or \code{'wb'}, depending on
+whether the file will be read or written.  The default is the mode of
+\var{fileobj} if discernible; otherwise, the default is \code{'rb'}.
+Be aware that only the \code{'rb'}, \code{'ab'}, and \code{'wb'}
+values should be used for cross-platform portability.
 
 The \var{compresslevel} argument is an integer from \code{1} to
 \code{9} controlling the level of compression; \code{1} is fastest and
@@ -55,7 +60,7 @@ object's \method{getvalue()} method.
 \begin{funcdesc}{open}{filename\optional{, mode\optional{, compresslevel}}}
 This is a shorthand for \code{GzipFile(\var{filename},}
 \code{\var{mode},} \code{\var{compresslevel})}.  The \var{filename}
-argument is required; \var{mode} defaults to \code{'r'} and
+argument is required; \var{mode} defaults to \code{'rb'} and
 \var{compresslevel} defaults to \code{9}.
 \end{funcdesc}