Added 'expiry' description

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@108 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

appendixa/shadowpwd-desc.sgml

@@ -42,7 +42,7 @@ chsh  changes the user login shell.
 <sect3><title>expiry</title>
 
 <para>
-It's currently unknown what this program is for.
+Checks and enforces password expiration policy.
 </para>
 
 </sect3>