Include an "Explanation or submission guidelines" in the basic settings, like with content types

An alternative would be to add a markup field to the feed entity type (because feeds_feed entities are fieldable), but I suppose we could add a field to the basic settings as well.

Sidenote: I did notice some validation issues when trying to import as an user with less privileges using the UI. This happens when you want your users to import content that they may not create manually. I've been working on fixing this in the case that feeds are created in advance by an admin user: #2811429: Switch to feed owner during manual import..

I haven't tried, but I think the routes start with entity.feeds_feed. and then need to be complemented with something that is listed on the "links" section in the annotation of the \Drupal\feeds\Entity\Feed class:

 *   links = {
 *     "canonical" = "/feed/{feeds_feed}",
 *     "add-page" = "/feed/add",
 *     "add-form" = "/feed/add/{feeds_feed_type}",
 *     "delete-form" = "/feed/{feeds_feed}/delete",
 *     "edit-form" = "/feed/{feeds_feed}/edit",
 *     "import-form" = "/feed/{feeds_feed}/import",
 *     "schedule-import-form" = "/feed/{feeds_feed}/schedule-import",
 *     "clear-form" = "/feed/{feeds_feed}/delete-items"
 *   }

Feeds uses the route provider "\Drupal\Core\Entity\Routing\AdminHtmlRouteProvider" to generate the routes for the feeds_feed entity type. That derives from \Drupal\Core\Entity\Routing\DefaultHtmlRouteProvider, which says in the docblock the following:

/**
 * Provides HTML routes for entities.
 *
 * This class provides the following routes for entities, with title and access
 * callbacks:
 * - canonical
 * - add-page
 * - add-form
 * - edit-form
 * - delete-form
 * - collection
 * - delete-multiple-form
 *
 * @see \Drupal\Core\Entity\Routing\AdminHtmlRouteProvider.
 */

If I look at the implementation of DefaultHtmlRouteProvider::getRoutes() I see underscores used instead of dashes:

    if ($add_form_route = $this->getAddFormRoute($entity_type)) {
      $collection->add("entity.{$entity_type_id}.add_form", $add_form_route);
    }

    if ($canonical_route = $this->getCanonicalRoute($entity_type)) {
      $collection->add("entity.{$entity_type_id}.canonical", $canonical_route);
    }

    if ($edit_route = $this->getEditFormRoute($entity_type)) {
      $collection->add("entity.{$entity_type_id}.edit_form", $edit_route);
    }

So I assume one of the routes must be entity.feeds_feed.edit_form, not entity.feeds_feed.edit-form.

Looks good to me except trailing whitespace errors

explanation-submission-guidelines-3080985-8.patch:99: trailing whitespace.
      '#description' => $this->t('This text will be displayed at the top 
Checking patch config/schema/feeds.schema.yml...
Checking patch feeds.module...
Checking patch src/Entity/FeedType.php...
Checking patch src/FeedTypeForm.php...
Hunk #1 succeeded at 127 (offset -1 lines).
Checking patch src/FeedTypeInterface.php...
Applied patch config/schema/feeds.schema.yml cleanly.
Applied patch feeds.module cleanly.
Applied patch src/Entity/FeedType.php cleanly.
Applied patch src/FeedTypeForm.php cleanly.
Applied patch src/FeedTypeInterface.php cleanly.
warning: 1 line adds whitespace errors.

@andileco
Looks straightforward. I tried the patch and it works. One thing I noticed is that caches need to be cleared before the help text is displayed, but this is also the case with content types in core. I tried also to fill in some javascript code to make sure it isn't executed: text is escaped like it should.

Committed #10 with one change:
I removed one extra use statement in feeds.module (see coding standards message on https://www.drupal.org/pift-ci-job/1410197).