Updated module documentation for clarification

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.
Merge pull request #15652 from loz-hurst/patch-1

Updated module documentation for clarification. fixes #15616