add note to serial docs

##### SUMMARY
<!--- Your description here -->
Add a link to the serial docs to explain scope of failure
Fixes #76820

##### ISSUE TYPE
- Docs Pull Request

+label: docsite_pr

docs/docsite/rst/user_guide/playbooks_strategies.rst

@@ -105,6 +105,9 @@ In the above example, if we had 6 hosts in the group 'webservers', Ansible would
     web6      : ok=2    changed=2    unreachable=0    failed=0
 
 
+.. note::
+   Setting the batch size with ``serial`` changes the scope of the Ansible failures to the batch size, not the entire host list. You can use  ``ignore_unreachable`` or :ref:`maximum_failure_percentage`.
+   
 You can also specify a percentage with the ``serial`` keyword. Ansible applies the percentage to the total number of hosts in a play to determine the number of hosts per pass:
 
 .. code-block:: yaml