Add note to serial docs (#77006)

* add note to serial docs
* add keyword directly
Fixes #76820
(cherry picked from commit de20f1bae8)

docs/docsite/rst/user_guide/playbooks_error_handling.rst

@@ -24,6 +24,8 @@ By default Ansible stops executing tasks on a host when a task fails on that hos
 
 The ``ignore_errors`` directive only works when the task is able to run and returns a value of 'failed'. It does not make Ansible ignore undefined variable errors, connection failures, execution issues (for example, missing packages), or syntax errors.
 
+.. _ignore_unreachable:
+
 Ignoring unreachable host errors
 ================================
 
@@ -145,7 +147,7 @@ Ansible lets you define when a particular task has "changed" a remote node using
 
     tasks:
 
-      - name: Report 'changed' when the return code is not equal to 2 
+      - name: Report 'changed' when the return code is not equal to 2
         ansible.builtin.shell: /usr/bin/billybass --mode="take me to the river"
         register: bass_result
         changed_when: "bass_result.rc != 2"

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  :ref:`ignore_unreachable <ignore_unreachable>` or :ref:`max_fail_percentage <maximum_failure_percentage>` to modify this behavior.
+
 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