Add info about loop based on jinja2 loop var (#42134)

* Add info about loop based on jinja2 loop var

* ansible_loop

* Update test count

* Add extended loop_control that defines whether ansible_loop should be added

* Extended needs to be defaulted

* Revert "Update test count"

This reverts commit f1e93ee469825f4cdcd90fb28667d29aa088275c.

* Add docs about loop_control.extended

* Add revindex and revindex0

* Document ansible_loop in special vars

* Add changelog fragment

* Add tests, change items to allitems so that dot notation works, fix logic error with previtem

docs/docsite/rst/user_guide/playbooks_loops.rst

@@ -330,6 +330,30 @@ If you need to keep track of where you are in a loop, you can use the ``index_va
       loop_control:
         index_var: my_idx
 
+.. versionadded:: 2.8
+
+As of Ansible 2.8 you can get extended loop information using the ``extended`` option to loop control. This option will expose the following information.
+
+==========================  ===========
+Variable                    Description
+--------------------------  -----------
+``ansible_loop.allitems``   The list of all items in the loop
+``ansible_loop.index``      The current iteration of the loop. (1 indexed)
+``ansible_loop.index0``     The current iteration of the loop. (0 indexed)
+``ansible_loop.revindex``   The number of iterations from the end of the loop (1 indexed)
+``ansible_loop.revindex0``  The number of iterations from the end of the loop (0 indexed)
+``ansible_loop.first``      ``True`` if first iteration
+``ansible_loop.last``       ``True`` if last iteration
+``ansible_loop.length``     The number of items in the loop
+``ansible_loop.previtem``   The item from the previous iteration of the loop. Undefined during the first iteration.
+``ansible_loop.nextitem``   The item from the following iteration of the loop. Undefined during the last iteration.
+==========================  ===========
+
+::
+
+      loop_control:
+        extended: yes
+
 Migrating from with_X to loop
 `````````````````````````````