systemd-cat: add examples (#17754)

* systemd-cat: add examples

Author: acuteenvy

pages/linux/systemd-cat.md

@@ -10,3 +10,15 @@
 - Write the output of a pipeline to the journal (`stderr` stays connected to the terminal):
 
 `{{command}} | systemd-cat`
+
+- Use the specified identifier (default: `cat` when reading from a pipeline, executable name otherwise):
+
+`{{command}} | systemd-cat {{[-t|--identifier]}} {{id}}`
+
+- Use the specified default priority level for all logged messages:
+
+`systemd-cat {{[-p|--priority]}} {{emerg|alert|crit|err|warning|notice|info|debug}} {{command}}`
+
+- Use the specified default priority level for the logged messages from the command's `stderr`:
+
+`systemd-cat --stderr-priority {{emerg|alert|crit|err|warning|notice|info|debug}} {{command}}`