udevd: update snippet string

Repeating the unit name in the description is not useful, and "manages devices"
is too cryptic.

src/udev/udevd.c

@@ -1472,7 +1472,7 @@ static int help(void) {
                 return log_oom();
 
         printf("%s [OPTIONS...]\n\n"
-               "Manages devices.\n\n"
+               "Rule-based manager for device events and files.\n\n"
                "  -h --help                   Print this message\n"
                "  -V --version                Print version of the program\n"
                "  -d --daemon                 Detach and run in the background\n"

units/systemd-udevd.service.in

@@ -8,7 +8,7 @@
 #  (at your option) any later version.
 
 [Unit]
-Description=udev Kernel Device Manager
+Description=Rule-based manager for device events and files
 Documentation=man:systemd-udevd.service(8) man:udev(7)
 DefaultDependencies=no
 After=systemd-sysusers.service systemd-hwdb-update.service