Update yum_repository.py (#42859)

* Update yum_repository.py

Having spent some time trying and googling how to enable an installed repo with this, I discovered that the thread at https://github.com/ansible/ansible-modules-extras/issues/2384 had decided that this would not be supported and recommended use of ini_file instead.  Since I am sure I'm not alone in expecting yum-repository to support enabling/disabling a configured repo, I suggest adding a note so people find this easier.

lib/ansible/modules/packaging/os/yum_repository.py

@@ -23,6 +23,7 @@ version_added: '2.1'
 short_description: Add or remove YUM repositories
 description:
   - Add or remove YUM repositories in RPM-based Linux distributions.
+  - If you wish to update an existing repository definition use M(ini_file) instead.
 
 options:
   async: