Add log file rotation section to howto.

Remove signals file, since the only signal you actually need to use is
for log-file rotation.

doc/ZEO/SIGNALS.txt

-Signals (POSIX only)
-
-  Signals are a POSIX inter-process communications mechanism.
-  If you are using Windows then this documentation does not apply.
-
-  The ZEO storage server process (started with ZEO/start.py) responds to
-  signals which are sent to the process id written to the file
-  'ZOPE_HOME/var/ZEO_SERVER.pid'::
-
-     SIGTERM - Close open storages and sockets, then shut down.
-
-               kill -TERM `cat ZOPE_HOME/var/ZEO_SERVER.pid`
-
-     SIGHUP  - Close open storages and sockets, then restart.
-
-               kill -HUP `cat ZOPE_HOME/var/ZEO_SERVER.pid`
-
-     SIGUSR2 - Rotate log files.
-
-               kill -USR2 `cat ZOPE_HOME/var/ZEO_SERVER.pid`
-
-  When using zdaemon (the default), the pid file contains the pid of
-  the zdaemon process.  Otherwise (when the -s option is used), the
-  pid file contains the storage server's own pid.
-
-  Note that zdaemon forwards signals to the child process.
-  Specifically, it forwards all those signals listed above, plus
-  SIGINT, SIGQUIT and SIGUSR1.

doc/ZEO/howto.txt

@@ -374,6 +374,16 @@ XXX example of how to use zdrun
 
 XXX mkzeoinst.py docs should probably go here
 
+Rotating log files
+~~~~~~~~~~~~~~~~~~
+
+ZEO will re-initialize its logging subsystem when it receives a
+SIGUSR2 signal.  If you are using the standard event logger, you
+should first rename the log file and then send the signal to the
+server.  The server will continue writing to the renamed log file
+until it receives the signal.  After it receives the signal, the
+server will create a new file with the old name and write to it.
+
 Diagnosing problems
 -------------------