doc: rewrite keventd documentation for unified device manager

Rewrite doc/keventd.md from a 14-line stub into comprehensive
documentation covering all features of the new unified keventd:
device node creation, persistent symlinks, firmware loading,
module loading, coldplug, conditions, and command-line usage.

Update doc/conditions.md to list keventd as the primary provider
of dev/* and sys/pwr/* conditions, with devmon as fallback when
an external device manager is used instead.

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

doc/conditions.md

@@ -177,15 +177,19 @@ Built-in Conditions
 
 Finit comes with a set of plugins for conditions:
 
- - `devmon` (built-in)
- - `netlink`
- - `pidfile`
+ - `keventd`: provides `<dev/...>` and `<sys/pwr/...>`
+ - `devmon` (built-in fallback for `<dev/...>` without keventd)
+ - `netlink`: provides `<net/...>`
+ - `pidfile`: provides `<pid/...>`
  - `sys`
  - `usr`
 
-The `devmon` (built-in) plugin monitors `/dev` and `/dev/dir` for device
-nodes being created and removed.  It is active only when a run, task, or
-service has declared a `<dev/foo>` or `<dev/dir/bar>` condition.
+The `dev/` conditions are provided by `keventd`, the built-in device
+manager.  When keventd creates a device node in `/dev`, it also asserts
+the corresponding `dev/` condition.  When a device is removed, the
+condition is cleared.  If keventd is not in use (an external device
+manager like udevd is used instead), the `devmon` built-in provides the
+same conditions by monitoring `/dev` and `/dev/dir` with inotify.
 
 The `pidfile` plugin recursively watches `/run/` for PID files created
 by the monitored services, and sets a corresponding condition in the