Review page-level help and make sure it's conceptual

On #2263359: hook_help(): Top of page help sections can't link to help pages without a fatal error or checking for help module → , we identified a problem (item 4 in the Problems section):

Most of the per-page help we have is stupid/useless. If the UI needs explaining, we need a different UI -- we shouldn't be explaining at the top of pages what the UI is about. And conceptual help like maybe "What is a forum container anyway" -- maybe that would be appropriate, but there's not a lot of that happening.

So, on this issue, the task is:

a) Go through the existing Core hook_help() (the page-level per-route help portions of it). Review the help being added to the System Help block on each page. If it is explaining concepts, keep it (or keep the portion that is); if it is explaining how to use the UI, get rid of it from the hook_help() implementation.

b) Review admin pages provided by Core modules to see if there are pages that refer to Drupal concepts that might not be obvious to a new Drupal user, and add help text to explain those concepts to the appropriate hook_help() implementation.