Clarify meaning of controller property.

Co-authored-by: Ted Thibodeau Jr <tthibodeau@openlinksw.com>
Co-authored-by: Ivan Herman <ivan.herman@me.com>
Co-authored-by: David Chadwick <d.w.chadwick@truetrust.co.uk>
Co-authored-by: Dave Longley <dlongley@digitalbazaar.com>