Add manual references & header to `nl`

sheets/nl

@@ -1,7 +1,10 @@
-# Number lines given to `nl` via STDIN.
+# nl
+# Number lines of files
+
+# Number lines given to nl(1) via STDIN.
 printf "Here\nis\nsome\nexample\ntext." | nl
 
-# Number (all) lines given to `nl` via provided file(s).
+# Number (all) lines given to nl(1) via provided file(s).
 nl -b a /path/to/file
-# The above can also be achieved with `cat`, which is perhaps more portable:
+# The above can also be achieved with cat(1), which is perhaps more portable:
 cat -n /path/to/file