docs: move config files + runtime dir doc to new configuration/preface

docs/usage.rst

@@ -1,3 +1,5 @@
+.. _usage:
+
 *****
 Usage
 *****
@@ -26,3 +28,33 @@ CLI Overview
     * - ``zrepl stdinserver``
       - see :ref:`transport-ssh+stdinserver`
 
+.. _usage-zrepl-daemon:
+
+============
+zrepl daemon
+============
+
+All actual work zrepl does is performed by a daemon process.
+Logging is configurable via the config file. Please refer to the :ref:`logging documention <logging>`.
+
+When installating from a package, the package maintainer should have provided an init script / systemd.service file.
+You should thus be able to start zrepl daemon using your init system.
+
+Alternatively, or for running zrepl in the foreground, simply execute ``zrepl daemon``.
+Note that you won't see much output with the :ref:`default logging configuration<logging-default-config>`:
+
+.. ATTENTION::
+
+    Make sure to actually monitor the error level output of zrepl: some configuration errors will not make the daemon exit.
+
+    Example: if the daemon cannot create the :ref:`transport-ssh+stdinserver` sockets in the runtime directory,
+    it will emit an error message but not exit because other tasks such as periodic snapshots & pruning are of equal importance.
+
+.. _usage-zrepl-daemon-restarting:
+
+Restarting
+~~~~~~~~~~
+
+The daemon handles SIGINT and SIGTERM for graceful shutdown.
+Graceful shutdown means at worst that a job will not be rescheduled for the next interval.
+The daemon exits as soon as all jobs have reported shut down.