perf: add link and -p example (#7229)

pages/linux/perf.md

@@ -1,6 +1,7 @@
 # perf
 
 > Framework for Linux performance counter measurements.
+> More information: <https://perf.wiki.kernel.org>.
 
 - Display basic performance counter stats for a command:
 
@@ -14,6 +15,10 @@
 
 `sudo perf record {{command}}`
 
+- Record the profile of an existing process into `perf.data`:
+
+`sudo perf record -p {{pid}}`
+
 - Read `perf.data` (created by `perf record`) and display the profile:
 
 `sudo perf report`