Sorting Album Photos by Date on my iPhone

You can only sort an album by dragging the photos manually around in the album. There is no automatic sort option.

The Help page for albums does not mention any sorting for the items in an album at all:

Organize photos in albums on iPhone - Apple Support

You are right. The user Guides are frequently incomplete. But I have not seen any workarounds posted here. My work-around is to sort all albums on my Mac and to sync the Photos Library on my Mac with the library on the iPhone with iCloud Photos. usually the albums will remain sorted like on my Mac. But that requires to have a Mac and to use iCloud Photos.