- (when ,ly:moment? "Global time step associated with this column
-happen?")
- (whiteout ,boolean? "If true, the grob is printed over a white
-background to white-out underlying material, if the grob is visible.
- Usually #f by default.")
+ (when ,ly:moment? "Global time step associated with this column.")
+ (whiteout ,boolean-or-number? "If a number or true, the grob is
+printed over a white background to white-out underlying material, if
+the grob is visible. A number indicates how far the white background
+extends beyond the bounding box of the grob as a multiple of the
+staff-line thickness. The @code{LyricHyphen} grob uses a special
+implementation of whiteout: A positive number indicates how far the
+white background extends beyond the bounding box in multiples of
+@code{line-thickness}. The shape of the background is determined by
+@code{whiteout-style}. Usually @code{#f} by default. ")
+ (whiteout-style ,symbol? "Determines the shape of the
+@code{whiteout} background. Available are @code{'outline},
+@code{'rounded-box}, and the default @code{'box}. There is one
+exception: Use @code{'special} for @code{LyricHyphen}.")