fixed issue with default callback inheritance (#30427)

* fixed issue with default callback inheritance

 - callbacks need to document same options as callbacks they inherit from to get them configured
 - since default is also used by many 3rd party callbacks for inheritance, making the code 'tolerate' the missing docs
   and fallback to using the direct constant to configure it's options.

lib/ansible/utils/module_docs_fragments/default_callback.py

@@ -0,0 +1,29 @@
+# (c) 2017 Ansible Project
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+
+class ModuleDocFragment(object):
+
+    DOCUMENTATION = """
+    options:
+      show_skipped_hosts:
+        name: Show skipped hosts
+        description: "Toggle to control displaying skipped task/host results in a task"
+        default: True
+        env:
+          - name: DISPLAY_SKIPPED_HOSTS
+        ini:
+          - key: display_skipped_hosts
+            section: defaults
+        type: boolean
+      show_custom_stats:
+        name: Show custom stats
+        description: 'This adds the custom stats set via the set_stats plugin to the play recap'
+        default: False
+        env:
+          - name: ANSIBLE_SHOW_CUSTOM_STATS
+        ini:
+          - key: show_custom_stats
+            section: defaults
+        type: bool
+"""