[PP-1] Move some detail from Package Manager's hook_help into a validator message, or a handbook page

Problem/Motivation

Spun off from ✨ Add Alpha level Experimental Package Manager module Needs review , specifically this MR comment.

Package Manager's hook_help implementation has a lot of detail in it. This is not a bad thing, but some of that detail might make more sense in the context of a message flagged by specific system validators, or maybe a handbook page that lives on drupal.org, which the validators or hook_help link to. Let's figure that out here.