Updated module documentation for clarification. fixes #15616

Added note to say that \__salt__ is not available to \__virtual__.  Also added link to 'at.py' as an example, since it demonstrates using salt.utils in \__virtual__ which the existing examples do not. Tweaked wording to make clear that not all the examples are package managers any more.

Fixed issue #15616 (https://github.com/saltstack/salt/issues/15616) as discussed in thread.