doc: add some info about keventd, what it does

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/conditions.md

@@ -87,12 +87,15 @@ Static (one-shot) conditions, like `usr/`, never enter the `flux` state.
 Built-in Conditions
 -------------------
 
-Finit comes with a `pidfile` and `netlink` plugin.  When enabled, the
-`pidfile` plugin watches `/var/run/` (recursively) for PID files created
-by the monitored services, and sets a corresponding condition in the
-`pid/` namespace.  Similarily, the `netlink` plugin provides basic
-conditions for when an interface is brought up/down and when a default
-route (gateway) is set, in the `net/` namespace.
+Finit comes with a set of plugins for conditions: `pidfile`, `netlink`,
+`sys`, and `usr`.  The `pidfile` plugin (recursively) watches `/run/`
+(recursively) for PID files created by the monitored services, and sets
+a corresponding condition in the `pid/` namespace.  Similarily, the
+`netlink` plugin provides basic conditions for when an interface is
+brought up/down and when a default route (gateway) is set, in the `net/`
+namespace.  The `sys` and `usr` plugins monitor are passive condition
+monitors where the action is provided by `keventd`, signal handlers,
+and in the case of `usr`, the end-user via the `initctl` tool
 
 With the example listed above, finit does not start the `/sbin/netd`
 daemon until `setupd` and `zebra` has started *and* created their PID
@@ -111,6 +114,8 @@ Built-in conditions:
 - `net/<IFNAME>/exist`
 - `net/<IFNAME>/up`
 - `net/<IFNAME>/running`
+- `sys/pwr/ac`
+- `usr/foo`
 
 **Note:** `up` means administratively up, the interface flag `IFF_UP`.
   `running` is the `IFF_RUNNING` flag, meaning operatively up.  The

doc/config.md

@@ -26,6 +26,7 @@ Configuration
   * [/etc/finit.conf](#etcfinitconf)
   * [/etc/finit.d](#etcfinitd)
 * [Watchdog](#watchdog)
+* [keventd](#keventd)
 
 
 Introduction
@@ -704,3 +705,19 @@ the only option is to remove `/libexec/finit/watchdogd` or build without
 it at configure time.
 
 [bootstrap]: bootstrap.md
+
+
+keventd
+-------
+
+The kernel event daemon bundled with Finit is a simple uevent monitor
+for `/sys/class/power_supply`.  It provides the `sys/pwr/ac` condition,
+which can be useful to prevent power hungry services like anacron to run
+when a laptop is only running on battery, for instance.
+
+Since keventd is not an integral part of Finit yet it is not enabled by
+default.  Enable it using `./configuure --with-keventd`.  The bundled
+contrib build scripts for Debian, Alpine, and Void have this enabled.
+
+This daemon is planned to be extended with monitoring of other uevents,
+patches and ideas are welcome in the issue tracker.