Document os.getenv().
This closes SF bug #429059.
diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index bc4d947..1767e37 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -166,6 +166,13 @@
Availability: \UNIX{}.
\end{funcdesc}
+\begin{funcdesc}{getenv}{varname\optional{, value}}
+Return the value of the environment variable \var{varname} if it
+exists, or \var{value} if it doesn't. \var{value} defaults to
+\code{None}.
+Availability: most flavors of \UNIX{}, Windows.
+\end{funcdesc}
+
\begin{funcdesc}{putenv}{varname, value}
\index{environment variables!setting}
Set the environment variable named \var{varname} to the string