"Explanation or submission guidelines" missing In editing a content type on a Drupal 8 site, I added text to the Explanation or submission guidelines, but when I surf to the node create/edit form, the text is nowhere to be seen.

Is this a bug, or are there extra steps involved that aren't documented in the UI?
That information is displayed in the help block. Make sure the theme you are using on those pages has a help block.

The Seven theme does have that by default, but either you've configured the node add/edit pages to use the front page theme, or you removed the help block.