Update apt.py (ansible#37972)

* Update apt.py

Proposing description of "cache_valid_time" for Ansible 2.4 or greater.
If 'cache_valid_time' is set, 'update_cache' is set implicitly.
This means the logic should be reversed.

 label: docsite_pr

* Proposing the description of "cache_valid_time"..

This commit follows the discussion in ansible#37972.

 label: docsite_pr

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

@@ -41,7 +41,7 @@
   cache_valid_time:
     description:
       - Update the apt cache if its older than the I(cache_valid_time). This option is set in seconds.
-        As of Ansible 2.4, this implicitly sets I(update_cache) if set.
+        As of Ansible 2.4, this sets I(update_cache=yes).
     default: 0
   purge:
     description: