doc: Augment release document.

* doc/release.org (Run “make release” for one or several release
candidates): Add example to gpg-agent TTL.
(Add a Git tag): Pass '-f' to autoreconf.

doc/release.org

@@ -39,7 +39,7 @@ your maintenance.git checkout.  Make sure to commit it:
 Create a signed Git tag, like this:
 
   $ git tag -s -u MY-KEY -m "GNU Guix X.YrcZ." vX.YrcZ
-  $ autoreconf  # to update $(PACKAGE_VERSION)
+  $ autoreconf -f  # to update $(PACKAGE_VERSION)
 
 Use “rcZ” for release candidates, drop it otherwise.
 
@@ -61,7 +61,15 @@ All these files will end up in a ‘release-X.Y.Z’ sub-directory.
 
 Needless to say, this operation takes some time.  Since this is
 committing on your behalf, it will contact your gpg-agent to sign
-commits, so you need to keep an eye on it.
+commits, so you need to keep an eye on it or to increase the
+time-to-live for cached passphrases in =~/.gnupg/gpg-agent.conf= (do
+that only if your machine is in a safe place!):
+
+#+begin_example
+  default-cache-ttl 36000
+  max-cache-ttl 36000
+
+#+end_example
 
 Steps #2 and #3 require you to have [[info:guix#Daemon%20Offload%20Setup][offloading]] set up so you can build
 for all the supported architectures.  For instance, if you’re running