Explain fetchTarball timeout behavior in the doc

doc/manual/expressions/builtins.xml

@@ -280,8 +280,17 @@ with import (fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixo
 
 stdenv.mkDerivation { … }
 </programlisting>
+    </para>
+
+    <para>The fetched tarball is cached for a certain amount of time
+    (1 hour by default) in <filename>~/.cache/nix/tarballs/</filename>.
+    You can change the cache timeout either on the command line with
+    <option>--option tarball-ttl <replaceable>number of seconds</replaceable></option> or
+    in the Nix configuration file with this option:
+    <literal>tarball-ttl <replaceable>number of seconds to cache</replaceable></literal>.
+    </para>
 
-    Note that when obtaining the hash with <varname>nix-prefetch-url
+    <para>Note that when obtaining the hash with <varname>nix-prefetch-url
     </varname> the option <varname>--unpack</varname> is required.
     </para>