documented new tag behaviour

docsite/rst/playbooks_tags.rst

@@ -40,6 +40,28 @@ And you may also tag basic include statements::
 
 Both of these have the function of tagging every single task inside the include statement.
 
+
+Special Tags
+````````````
+
+There is a special 'always' tag that will always run a task, unless specifically skipped (--skip-tags always)
+
+Example::
+
+    tasks:
+
+        - debug: msg="Always runs"
+          tags:
+            - always
+
+        - debug: msg="runs when you use tag1"
+          tags:
+            - tag1
+
+There are another 3 special keywords for tags, 'tagged', 'untagged' and 'all', which run only tagged, only untagged
+and all tasks respectively. By default ansible runs as if --tags all had been specified.
+
+
 .. seealso::
 
    :doc:`playbooks`