lineinfile - Add alternative backrefs example to docs (#69844)

* lineinfile - Add alternative backrefs example to docs

* Add changelog

changelogs/fragments/lineinfile-backrefs-syntax-example.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - lineinfile - add example of using alternative backrefs syntax (https://github.com/ansible/ansible/issues/42794)

lib/ansible/modules/lineinfile.py

@@ -200,6 +200,14 @@ EXAMPLES = r'''
     regexp: '^%ADMIN ALL='
     line: '%ADMIN ALL=(ALL) NOPASSWD: ALL'
     validate: /usr/sbin/visudo -cf %s
+
+# See https://docs.python.org/3/library/re.html for further details on syntax
+- name: Use backrefs with alternative group syntax to avoid conflicts with variable values
+  lineinfile:
+    path: /tmp/config
+    regexp: ^(host=).*
+    line: \g<1>{{ hostname }}
+    backrefs: yes
 '''
 
 import os