doc: describe the udev rules keventd actually implements

The engine covers most of the udev grammar but not all of it, and the
gaps are invisible until a rule silently does nothing.  Write down what
is implemented and where it parts ways with udev(7), rather than
leaving people to infer it from a ruleset that happens to work.  The
man page gets the directory precedence and a pointer to udev(7) and
the User's Guide, which hold the details.

Rename the menu entry to Device Manager while here, matching how the
watchdog daemon is listed.

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

mkdocs.yml

@@ -55,7 +55,7 @@ nav:
     - Runparts & rc.local: runparts.md
     - Plugins: plugins.md
     - Watchdog: watchdog.md
-    - keventd: keventd.md
+    - Device Manager: keventd.md
     - D-Bus Integration: dbus.md
     - Service State Machine: state-machine.md
     - Distributions: distro.md