added version added + union, diff and sym_diff filters

Signed-off-by: Brian Coca <briancoca+dev@gmail.com>

docsite/rst/playbooks_variables.rst

@@ -171,17 +171,28 @@ The variable value will be used as is, but the template evaluation will raise an
 
 Set Theory Filters
 --------------------
-These mostly deal with set (lists), no filter for union as this already works::
+All these functions return a unique set from sets or lists.
+.. versionadded:: 1.4
 
-    {{ list1 + list2 }}
+To get a unique set from a list::
+
+    {{ list1 |unique }}
+
+To get a union of two lists::
+
+    {{ list1 | union(list2) }}
 
 To get the intersection of 2 lists (unique list of all items in both)::
 
     {{ list1 |intersect(list2)}}
 
-To get a unique set from a list::
+To get the difference of 2 lists (items in 1 that don't exist in 2)::
 
-    {{ list1 |unique }}
+    {{ list1 |difference(list2)}}
+
+To get the symetric difference of 2 lists (items exclusive to each list)::
+
+    {{ list1 |symetric_difference(list2)}}
 
 .. _other_useful_filters: