Update yum.py

adding explanation why you must use "shell: yum clean all" and why it won't be included in the yum module as per ansible#31450 (comment)

+label: docsite_pr

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

@@ -209,6 +209,9 @@
     "@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.'
+  - use 'shell: yum clean all' - there is no other way to clear the cache and its 
+    been decided to not implement it as part of the module. 
+    https://github.com/ansible/ansible/pull/31450#issuecomment-352889579
 # informational: requirements for nodes
 requirements:
 - yum