-UNDERLINE non-nil means FACE explicitly does underlining
-with the same of the foreground color.
-If UNDERLINE is a string, underline with the color named UNDERLINE.
+UNDERLINE t means FACE underlines with its foreground color.
+If UNDERLINE is a string, underline with that color.
+
+UNDERLINE may also be a list of the form (:color COLOR :style STYLE),
+where COLOR is a string or `foreground-color', and STYLE is either
+`line' or `wave'. :color may be omitted, which means to use the
+foreground color. :style may be omitted, which means to use a line.
+