It would be about as hard to memorize \describe-schemas as it is to memorize \dn: You'd have to remember that it is "-" and not "_", that it is "describe", not "desc" and that it is "schemas", not "schema".
You wouldn't memorize them. You'd discover them with tab completion.
which is more heat than light. Yes, those are all the possibilites, but I, Joe Newguy, want to list schemas, and \ds and \dS look like the good guesses, neither of which is the right answer. If, with this feature, I typed \desc<tab>, I might see:
So my voyage of discovery would have completed with having typed "\desc<tab>-sc<tab>" and if we add a note to interactive mode, I'd be shown the hint that \dn is the shortcut for that just above the list of schemas.