Update usage documentation to include 'diff' in set operation options

Author: nmsaguiar

data/usage.json

@@ -1155,7 +1155,7 @@
     {
       "Option": "setop",
       "Type": "String",
-      "Description": "Allows to choose a different set operation between 'union', 'diffa', 'diffb', 'diffab' (symetric difference) and 'intersect' (default)"
+      "Description": "Allows to choose a different set operation between 'union', 'diffa', 'diffb', 'diffab' (symetric difference), 'diff' and 'intersect' (default)"
     }
   ],
   [

src/docs/USAGE.md

@@ -600,9 +600,10 @@ List of options to use when _set=..._:
 
 | Option | Type | Description |
 |--------|------|-------------|
-| setop | String | Allows to choose a different set operation between 'union', 'diffa', 'diffb', 'diffab' (symetric difference) and 'intersect' (default) |
+| setop | String | Allows to choose a different set operation between 'union', 'diffa', 'diffb', 'diffab' (symetric difference), 'diff' and 'intersect' (default) |
 
 > Example: ```set="(a: old, b: new)" setop=diffb```
+> 'setop=diff' will add an extra column '*' to identify if a line only exists in 'a' or in 'b'
 
 ---