Documentation update for kubernetes.core.helm
Clarify usage of the module for doing helm repo update only.
I used collection_prep_add_docs as explained in CONTRIBUTING.md, not sure if
that's correct ?
Fixes#316
@Akasurde
Reviewed-by: Abhijeet Kasurde <None>
Reviewed-by: Mike Graves <mgraves@redhat.com>
Reviewed-by: None <None>