Update docs with rhsm note (#48928)

* Update docs with rhsm note

redhat_subscription is the module for handling registration of modern Red Hat systems. This commit adds a note indicating that unless people are using RHEL 5 etc, they probably want that module instead.

+label: docsite_pr

* Make redhat_subscription a link to the module docs

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

@@ -22,6 +22,7 @@ description:
 version_added: "1.2"
 author: James Laska (@jlaska)
 notes:
+    - This is for older Red Hat products. You probably want the M(redhat_subscription) module instead.
     - In order to register a system, rhnreg_ks requires either a username and password, or an activationkey.
 requirements:
     - rhnreg_ks