change generated playbooks_keywords.rst to use an rst 'glossary' (#28843)

* Use a rst glossary for playbooks_keywords docs
* Add a 'Task' and 'Tasks' to glossary.
* Update keywords desciptions,
* use :term: rst ref, some quoting
* Make it more obvious that 'retries' and 'until' need to be used in combination.

docs/docsite/rst/glossary.rst

@@ -436,7 +436,7 @@ when a term comes up on the mailing list.
         labeled ``ntp``, and then run just the ``ntp`` steps to reconfigure
         the time server information on a remote host.
 
-    Tasks
+    Task
         :term:`Playbooks` exist to run tasks.  Tasks combine an :term:`action`
         (a module and its arguments) with a name and optionally some other
         keywords (like :term:`looping directives <loops>`).   :term:`Handlers`
@@ -444,6 +444,9 @@ when a term comes up on the mailing list.
         unless they are notified by name when a task reports an underlying
         change on a remote system.
 
+    Tasks
+        A list of :term:`Task`.
+
     Templates
         Ansible can easily transfer files to remote systems but often it is
         desirable to substitute variables in other files.  Variables may come