tac: add stdin example (#3844)

pages/common/tac.md

@@ -1,11 +1,15 @@
 # tac
 
-> Print and concatenate files in reverse.
+> Print and concatenate files in reverse (last line first).
 
 - Print the contents of *file1* reversed to the standard output:
 
 `tac {{file1}}`
 
+- Print the contents of the standard input reversed to the standard output:
+
+`{{command}} | tac`
+
 - Concatenate several files reversed into the target file:
 
 `tac {{file1}} {{file2}} > {{target_file}}`