documenting builtins

This pull request expands documentation of builtins and macros. Check especially that `require` is documented correctly.

Some builtins and macros are missing. especially `eval`, `eval-and-compile` and `eval-when-compile` are builtins which I don't know how to use properly. Any tips would be appreciated.

`defn` / `defun` and `let` I'll add as soon as I manage to write something about them.

When @Willyfrog PR #262 is accepted, I'll squash and rebase this on top of that.