Add note to apt_key doc about update requirement (#25111)

* Add note about adding new key requiring update

When a new repo and new key are added, attempts to install packages
signed by that key fail until `apt-get update` is run.  This note
is an attempt to help users avoid getting errors when they miss
this step.

* related to issue #25091

* switch example to apt module

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

@@ -26,6 +26,7 @@ notes:
     - Doesn't download the key unless it really needs it.
     - As a sanity check, downloaded key id must match the one specified.
     - Best practice is to specify the key id and the URL.
+    - Adding a new key requires an apt cache update (e.g. using the apt module's update_cache option)
 options:
     id:
         description: