Update unique filter docs (#84078)

* Correct case_sensitive example
* Add attribute docs

Signed-off-by: Abhijeet Kasurde <akasurde@redhat.com>

lib/ansible/plugins/filter/unique.yml

@@ -14,6 +14,9 @@ DOCUMENTATION:
       description: Whether to consider case when comparing elements.
       default: false
       type: bool
+    attribute:
+      description: Filter objects with unique values for this attribute.
+      type: str
   seealso:
     - plugin_type: filter
       plugin: ansible.builtin.difference
@@ -30,12 +33,25 @@ EXAMPLES: |
   # => [1, 2, 5, 3, 4, 10]
 
   # return case sensitive unique elements
-  {{ ['a', 'A', 'a'] | unique('case_sensitive=true') }}
+  {{ ['a', 'A', 'a'] | unique(case_sensitive='true') }}
   # => ['a', 'A']
 
   # return case insensitive unique elements
   {{ ['b', 'B', 'b'] | unique() }}
   # => ['b']
+
+  # return unique elements of list based on attribute
+  # => [{"age": 12, "name": "a" }, { "age": 14, "name": "b"}]
+  - debug:
+      msg: "{{ sample | unique(attribute='age') }}"
+    vars:
+      sample:
+        - name: a
+          age: 12
+        - name: b
+          age: 14
+        - name: c
+          age: 14
 RETURN:
   _value:
     description: A list with unique elements, also known as a set.