Do you know about AutoGeneratePackage? Either code below will save the cells marked InitializationCell (see menu Cell > Cell Properties > Initialization Cell, shortcut ctrl-8):

SetOptions[EvaluationNotebook[], AutoGeneratedPackage -> Automatic]
SetOptions[EvaluationNotebook[], AutoGeneratedPackage -> Manual]

See the documentation for the difference between Automatic and Manual. You could also look at the code for GeneralUtilities`PrintDefinitions (via GeneralUtilities`PrintDefinitions[GeneralUtilities`PrintDefinitions]), which prints code to a notebook. It may take some work to figure it out, since it does a lot of pretty-print formatting with hyperlinks. It's much more complicated than the basic task you want to do.

Here's a usage string generator similar to what you want:

ClearAll[fff];
fff[x_, y_ : 2] := x + y;
fff[x_, y_, z_] := x  z + y;

fff::usage = StringJoin@ Riffle[ToString /@ HoldForm @@@ DownValues[fff][[All, 1]], "\n"];
? fff

A more complicated example:

ClearAll[fn];
fn[e_, {x_, a_, b_}][x0_?NumericQ] := Block[{x = x0}, e];
fn[e_, {x_, a_, b_}]["Domain"] := {x, a, b};
fn[e_, {x_, a_, b_}]["Variable"] := x;
fn[e_, x_Symbol] := fn[e, {x, 0, 1}]; (* default domain *)
fn /: Plot[f_fn] := 
  With[{x = f["Variable"], domain = f["Domain"]}, Plot[f[x], domain]];

fn::usage = StringJoin@ Riffle[
   ToString /@ HoldForm @@@ Join[UpValues[fn], SubValues[fn], DownValues[fn]][[All, 1]], 
    "\n"];

One could also just write a utility function to use on functions under development:

usage[symb_] := HoldForm @@@ 
    Join[UpValues[symb], SubValues[symb], DownValues[symb]][[All, 1]] // 
   Column;