Update documentation before updating API

There are two major differences in the next version of the API:

* in the sequence of parameters for the callback, publish and subscribe
  now come before get and set. I have noticed while using the API that it
  is somewhat more common to have modules using only publish/subscribe rather
  than only get/set. Defining them first allows to omit unused parameters.

* the space returned by within(name) and within() without argument is now
  a function which can be called to run code within the space. This addition
  addresses the lack of a method to access 'this' with an anonymous space,
  resulting in more tedious code to assign multiple properties separately
  with set():

        var anonymous = space();
        space(function() {
          this.a = 1;
          this.b = 2;
          this.c = 3;
          ...
        });

  vs

        var anonymous = space();
        // duplicate calls to space.set(), property name must be quoted
        space.set('a', 1);
        space.set('b', 2);
        space.set('c', 3);
        ...

I reordered descriptions to align the presentation of the methods
with the new order of the parameters.

I also added examples to illustrate explanations where missing,
and I removed references of spaces as 'modules', using the term
'space' more consistently instead.