poweroff, halt, reboot: add examples (#5485)

pages/linux/poweroff.md

@@ -1,7 +1,24 @@
 # poweroff
 
-> Shutdown the system.
+> Power off the system.
+> More information: <https://www.man7.org/linux/man-pages/man8/poweroff.8.html>.
 
-- Poweroff the system:
+- Power off the system:
 
-`sudo poweroff`
+`poweroff`
+
+- Halt the system (same as `halt`):
+
+`poweroff --halt`
+
+- Reboot the system (same as `reboot`):
+
+`poweroff --reboot`
+
+- Shut down immediately without contacting the system manager:
+
+`poweroff --force --force`
+
+- Write the wtmp shutdown entry without shutting down the system:
+
+`poweroff --wtmp-only`