I've wondered about that for years and never succeeded in getting Hmisc::describe to honor that parameter (despite using Hmisc extensively). Here's what I would do to get something similar:
print(rbind( count = as.character(table(letters[1:20])),
pct = 100*prop.table(table(letters[1:20]) ) ),
quote=FALSE)
#-----------------------------
a b c d e f g h i j k l m n o p q r s t
count 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
pct 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
You may want to format() the pct to round to a certain number (0 or 1) of decimal places.