doc: Add module-level comments for functions.

scribblings/functions.scrbl

@@ -1,12 +1,19 @@
 #lang scribble/manual
-@(require (for-label racket graph))
+@(require (for-label racket "../functions.rkt"))
 
 @title[#:tag "functions"]{dds/functions: Formal Functions}
 
-@racketblock[
-(define (nobody-understands-me what)
-  (list "When I think of all the"
-        what
-         "I've tried so hard to explain!"))
-(nobody-understands-me "glorble snop")
-]
+@defmodule[dds/functions]
+
+This modules provides some definitions for working with functions: tabulating,
+(re)constructing from tables, generating random functions, etc.
+Some definitions of particular kinds of functions are also provided (threshold
+Boolean functions, etc.).
+
+@section{Tabulating functions}
+
+@section{Constructing functions}
+
+@section{Random functions}
+
+@section{Threshold Boolean functions}