pkill: add --oldest example and link (#5445)

pages/common/pkill.md

@@ -2,15 +2,20 @@
 
 > Signal process by name.
 > Mostly used for stopping processes.
+> More information: <https://www.man7.org/linux/man-pages/man1/pkill.1.html>.
 
 - Kill all processes which match:
 
-`pkill -9 {{process_name}}`
+`pkill -9 "{{process_name}}"`
 
 - Kill all processes which match their full command instead of just the process name:
 
-`pkill -9 -f "{{command_name}}"`
+`pkill -9 --full "{{command_name}}"`
 
 - Send SIGUSR1 signal to processes which match:
 
-`pkill -USR1 {{process_name}}`
+`pkill -USR1 "{{process_name}}"`
+
+- Kill the main `firefox` process to close the browser:
+
+`pkill --oldest "{{firefox}}"`