tail: add example using -s parameter. (#4180)

pages/common/tail.md

@@ -21,3 +21,7 @@
 - Keep reading file until `Ctrl + C`, even if the file is rotated:
 
 `tail -F {{file}}`
+
+- Show last 'num' lines in 'file' and refresh every 'n' seconds:
+
+`tail -n {{num}} -s {{n}} -f {{file}}`