Update yum.py (#47749)

add explanation why you must use "shell: yum clean all" and why it won't be included in the yum module as per https://github.com/ansible/ansible/pull/31450#issuecomment-352889579

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

@@ -210,6 +210,10 @@ notes:
     "@development-tools" and environment groups are "@^gnome-desktop-environment".
     Use the "yum group list" command to see which category of group the group
     you want to install falls into.'
+  - 'The yum module does not support clearing yum cache in an idempotent way, so it
+    was decided not to implement it, the only method is to use shell and call the yum
+    command directly, namely "shell: yum clean all"
+    https://github.com/ansible/ansible/pull/31450#issuecomment-352889579'
 # informational: requirements for nodes
 requirements:
 - yum