Actually, it's not undocumented at all.
It's still there.
The HELP file for the guitar window states:

"These buttons will chord step advance [->], or note-step advance [>]. The chord step advance is the most commonly used function. It is also accessible by the hot keys Ins and Del on the numeric keypad and will advance or go back one chord at a time, leaving the chord displayed on the guitar."

If the melody is single note, the Ins & Del keys will sound one note at a time. If there are chords, it will sound those instead.

LLOYD S