doc: Document how to manually shut down gpg-agent.

* doc/gpg-agent.texi: document "gpgconf --kill gpg-agent" for manual
  agent termination.

This was requested in a side-comment in https://bugs.debian.org/840669

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

doc/gpg-agent.texi

@@ -84,6 +84,14 @@ the included Secure Shell Agent you may start the agent using:
 gpg-connect-agent /bye
 @end example
 
+@noindent
+If you want to manually terminate the currently-running agent, you can
+safely do so with:
+
+@example
+gpgconf --kill gpg-agent
+@end example
+
 @noindent
 @efindex GPG_TTY
 You should always add the following lines to your @code{.bashrc} or
@@ -1518,6 +1526,7 @@ much slower or faster than the actual box.
 @ifset isman
 @command{@gpgname}(1),
 @command{gpgsm}(1),
+@command{gpgconf}(1),
 @command{gpg-connect-agent}(1),
 @command{scdaemon}(1)
 @end ifset